Mina had been in enough old buildings to know that they breathed differently at night.

She had grown up sleeping in Agnes's spare room on the weekends she stayed over, long before the bookshop became something she visited with a key and a quiet grief. The building had been old, genuinely old, with plumbing that thought about itself audibly and floors that had opinions about temperature, and she had learned very early to distinguish between the ordinary sounds of an old building settling and the sounds that meant something else.

Cassian's bookshop made sounds she had never heard before.

She lay on her back in the fourth-floor room and listened to the shop at eleven-forty in the evening and catalogued what she heard. The ordinary sounds were ordinary: wood contracting in the cool, water in the pipes somewhere below, the faint sounds of the street outside and the distant bell that marked the hours. She had identified all of these and filed them as explained.

The unexplained sounds were: a soft resonance, below hearing rather than at it, felt in the chest more than the ears, as though the shop were producing a frequency in a register designed to be noticed only when everything else was quiet. And occasionally, very occasionally, something that might have been a book moving, the faint slide of a spine along a shelf, though she could not have sworn to this. And once, clearly, a sound she could only describe as the sound of a room changing its mind.

She was a practical person. She got up.

The staircase was dark but she had memorized it over four days of use, and she went down to the ground floor in bare feet, in the oversized shirt she had borrowed from the spare folded pile in the hall cupboard, and she stood at the bottom of the stairs and looked at the shop.

The shop was different.

Not dramatically, not in the way of horror stories, but in the way of a room rearranged by someone with an opinion about furniture: the shelving on the left wall had shifted, she was nearly certain, perhaps six inches clockwise, which had the effect of opening a narrow passage between two sections that had previously been nearly touching. And at the back of the shop, where there had been a solid wall with a small window, there was, now, a door.

Not the kind of door that appeared out of nowhere and glowed and took you to other worlds. A plain door, wooden, old, slightly ajar, with darkness beyond it.

She went toward it.

The darkness behind the door was cold and smelled of paper, and when she put her hand in and found a lamp and turned it on, she saw the room.

It was small, rectangular, low-ceilinged, and it was full of books that were not in the main shop's catalog. She could tell this immediately because the books in the main shop were organized by Cassian's system, carefully shelved and labeled, while the books in this room were organized by a different system entirely: uniform in size and binding, row upon row of identical volumes, dark brown leather, each one marked only with a small symbol on the spine that she did not recognize. Dozens of them. Possibly more than a hundred.

And every single one of them was blank.

She took the nearest one from the shelf and opened it. Blank. The next one: blank. She moved along the shelf, pulling volumes at random, finding the same thing: empty page after empty page, the same high-quality paper with the same smell, the same slight weight, the same warmth through the leather when she held them.

The same warmth as the book Agnes had left her.

She took one and opened it to the first page and put her hand flat on the paper.

The resonance that came back was not like anything she had read from other objects. Not an Afterglow, not an emotional impression, not the layered sediment of past readers. What she felt was potential: a quality of readiness, of capacity, of something that was entirely and specifically not-yet. A before. The book was a before.

She pulled her hand back and looked at it in the yellow lamp light.

On the inside of the front cover, in handwriting she had known since childhood, faded but unmistakably Agnes's: For when you're ready to go back.

She stood very still for a long time.

She had seen Agnes's handwriting on birthday cards, on the small paper bags that held the books her grandmother set aside for her, on the labels in the shop's catalog, on the back of a photograph that Mina kept in her wallet. She had not expected to see it here, three streets from a railway station in a kingdom she had arrived in through a door that should not have existed, on the inside cover of a book that felt exactly like the book that had brought her here.

She sat down on the cold floor because it seemed like the appropriate response to a revelation.

She was still sitting there when the door opened.

Cassian filled the doorway. He was still dressed, which surprised her, and he was looking at her on the floor with the volume in her hands, and his expression moved through several stages in rapid succession before settling on something she could not categorize.

He said: "Put the book down and come out of there."

He did not say it with anger. He said it with something more complicated than anger, something that had more components, and the component she identified most clearly, the one that sat at the front of his voice, was fear.

She looked at the book in her hands, then up at him.

"My grandmother wrote in this," she said. "Her handwriting. I know it as well as I know my own."

He said nothing. The lamp between them made his expression harder to read, putting parts of his face in shadow.

"Agnes Whitmore knew about this room," she said. "About these books." She paused. "About Avelune."

Still nothing.

"This is not the first time a blank book from this room opened a door for someone," she said.

He stepped into the room. He moved to the shelf and looked at it, at the row of books she had pulled from it, and he was quiet in the way that he was quiet when he was holding himself very still because the alternative was something he was not ready for.

"Cassian," she said.

He turned and looked at her.

"Yes," he said. "Your grandmother knew about Avelune."

She had expected this. She had known it from the moment she had recognized the handwriting. Knowing and having it confirmed were different things, she was learning, and the difference lived in the chest.

She looked down at the book in her hands. "She left this for me," she said. "Specifically. She left it in the shop she knew I would come back to, because she knew I would keep coming back, because this is what I do, I come back to the places she left for me." She heard her own voice doing something that was not crying but was adjacent to it and she steadied it. "She knew I would find this book."

"Yes," he said, very quietly.

"Did she know it would bring me here?"

A pause longer than any she had catalogued before.

"I believe she knew it would be possible," he said. "Whether she knew it was certain, I cannot say."

She looked up at him. He was standing in the room with her now, close enough that she could see his face clearly, close enough that the lamp made them both part of the same circle of light.

"What was she to you?" she asked.

He met her eyes. He held the look for a long moment, and she saw him decide, very deliberately, to give her something real.

"She was someone who understood what I was looking for," he said. "And who believed it was worth finding."

She absorbed this. She was going to need more than this, she was going to need a great deal more than this, but she understood that this was the version of the truth available to him tonight, in this cold room, with her sitting on the floor holding a book with her grandmother's handwriting in it.

She stood up.

He took a half-step back, giving her room. She appreciated this.

She put the book back on the shelf.

"Tomorrow," she said, "you are going to tell me everything you know about my grandmother and Avelune."

"Yes," he said.

"Tonight," she said, "I am going to go back to bed."

She moved toward the door. He stepped aside to let her pass, which was the obvious thing to do and also, she noticed, something he did without hesitation, which was a different kind of obvious.

At the threshold she stopped.

"She said to come back," she said, without turning. "In the inscription. She said, for when you are ready to go back. I had thought she meant to my writing, or to myself, or something abstract like that." She paused. "She meant here."

"Yes," he said. Quietly, but with something in it that sounded like relief: not that she was hurt by this, but that she had understood it, which were different kinds of relief and the second was the more careful kind.

"I am not angry," she said.

"I know," he said.

She did not know how he knew, but she believed him. She went upstairs and lay down on the bed in the dark and held the leather-bound book, her book, the one she had been writing in, and she thought about Agnes with her system of emotional neighborhoods and her spare room and her birthday-card handwriting and her deliberate, patient, long-prepared welcome.

She thought: you knew I would need a door.

She thought: you built me one.

She fell asleep with her hand flat on the cover of the book, the warmth of it against her palm, and if she cried at all in the dark before sleeping she was not going to mention it to anyone, because some things were private even from the narrator.

She did not go back to the locked room the following day.

She was aware that she could: she knew where it was now, she knew how the shop rearranged itself to open the passage, she knew that the room existed and what was in it and she could, in theory, return whenever the shop chose to make it accessible again. She chose not to, which was its own kind of decision, a deliberate decision to let a thing sit for a day rather than going at it immediately, which was not her natural approach but which she was practicing.

Cassian said nothing about the previous night.

This was not the same as avoiding it. He made breakfast with the particular care that she had come to understand was his version of acknowledgment: the tea at the right temperature, an extra slice of bread, the precision that said I know last night happened and I am treating this morning with appropriate attention without requiring you to discuss it.

She ate the bread and drank the tea and looked at the list on the worktable.

"I want to go back to the room," she said, eventually.

He looked up from his ledger.

"I want to read the books," she said. "The blank ones. I want to understand what they are, why they exist here in this quantity, what Agnes was doing with them or what they were doing with Agnes." She paused. "I think it is relevant to the case."

He set the ledger down.

"They were made here," he said. "In this building. Not by me: my family made them, over generations, for purposes connected to the Afterglow archive." He paused. "A blank book prepared in this way is a vessel. It can hold Afterglow, or hold a fragment, or hold a record of something that needs to be preserved outside the official system." He looked at the wall. "The room is not mine. It came with the building when I inherited the shop."

"Your family owned this building?"

"For three generations," he said. "It was an archive annexe before it was a bookshop. My grandfather converted it."

She thought about the room full of blank books and the Afterglow of potential she had felt from them, the readiness of them, the quality of capacity. "They are waiting," she said. "To be written in."

"Yes," he said.

"Agnes wrote in one," she said. "The one she left for me."

"Yes," he said.

She looked at him. "Why was her book here? In your family's archive?"

He was quiet for a long moment.

"Because your grandmother," he said, carefully, "was the person who first showed me the fragment in my Afterglow." He paused. "She was the archivist who found it. She brought it to my attention when I was nineteen."

The morning was very still.

"She was working here," Mina said. "In Avelune."

"She came and went for many years," he said. "She had a connection to the archive, a professional one, a relationship between her bookshop and the archive's collection. She had a gift for finding lost things." He paused. "Including lost Afterglow."

She looked at the wall where the door to the hidden room would appear tonight.

She thought about Agnes with her system of emotional neighborhoods, her talent for finding exactly the right book for exactly the right reader, her inexplicable certainty about what a person needed even when they had not said so. She thought about Agnes who had kept blank books in a building that turned out to be connected to the most significant thing in Cassian Vey's life. She thought about a woman who had lived between two worlds and sent her granddaughter through a door she had kept ready.

"She knew you," Mina said.

"Yes," he said.

"She liked you," Mina said, and this was not a question, because she knew the answer, she could feel it in the shape of everything Agnes had done, the preparation and the patience and the inscription: for when you are ready to go back.

"She was," he said, and paused, and said the thing she did not expect: "kind to me, at a time when I had not been kind to myself."

The morning settled around this.

She did not press it. She had the shape of it, which was enough: a young man at nineteen who had found something terrible and made a choice and been met with kindness from a woman who understood more than she let on, a woman who had then spent years keeping a door ready for the person who might be able to help him find what he had lost.

"She sent me here," Mina said.

"I believe so," he said.

"On purpose."

"I believe that too."

She looked at the room around her, at the books and the lamps and the list on the worktable and the two cups on the shelf with different amounts of tea left in them.

She thought: Agnes, you absolute schemer.

She thought: thank you.

She stood up and took her cup and rinsed it in the kitchen and set it on the shelf, and when she came back he was looking at the ledger again but she could see he was not reading it.

She said: "We go to the archive tomorrow."

"Yes," he said.

"And tonight I go back to the room and I read the books properly."

A pause. "Yes," he said. "If you want to."

"I want to," she said.

She went to the shelves and began the day's work, and behind her the shop breathed around them, and somewhere in the hidden room the blank books waited with their quality of capacity, and the morning passed the way mornings were passing: in the particular texture of two people sharing a space in which something was being built.

She could feel it being built. She was not going to say so yet.

But she could feel it.

The hidden room was different in the morning.

She went back to it when the shop made the passage available, which was always in the early evening, but she had been thinking about it all day and she returned with more intention than she had the previous night. The lamp was where she had left it. The blank books were where they had always been.

She worked through them methodically.

Most of them gave her the same reading: capacity, potential, readiness. The feeling of a vessel designed to hold something that had not yet been put in it. She had read eleven of them this way before she found the one that was different.

It was on the second shelf, third from the right, and she might have missed it if she had not been going through them systematically. From the outside it was identical to the others: same binding, same size, same small symbol on the spine. From the inside it was entirely different.

The book had been written in.

Not in ink, not in pencil, not in any visible way: she could open it and find blank pages the same as the others. But when she held it and put her flat hand on the first page, the resonance was not capacity and potential. It was presence and record, the feeling of a book that had been used for exactly the purpose it was made for.

She stayed with it for a long time, longer than she had stayed with any single object in Avelune.

What she got from it was not images, not the visual kind, not the Afterglow kind. It was more like language, like being told something rather than shown it. Not words she could repeat: impression, the way you got the sense of a thing that had happened without being able to report the specifics. The impression was: a record of something that needed to be preserved outside the official system. The impression was: deliberate, careful, chosen. The impression was: kept for a purpose that was not yet complete.

She set the book back on the shelf.

She looked at it for a long time.

Then she went upstairs and knocked on the door of the back room, which was the first time she had knocked rather than calling from the hall.

He came to the door.

She looked at him.

"There is a book in that room," she said, "that has been written in. Not in any way I can read with my eyes. But it has been used. Someone put something in it." She paused. "I believe it was my grandmother."

He was very still.

"I believe so too," he said. "I have not been able to read it."

"But I can feel that it is there," she said.

"Yes," he said. "I know."

"Then we need to find a way to read it," she said. "That is what we do next."

He looked at her, and the expression on his face in the lamplight from the hallway was the most unguarded thing she had seen on him, and what it was, she thought, was hope. Not hope in the large and impossible sense but the specific practical hope of a person who has been working toward a thing for a long time and has just been given a reason to believe it is possible.

She had given him that.

She was not going to examine what that felt like right now. Later. Later was still a useful container.

"Tomorrow," she said.

"Tomorrow," he agreed.

She went to her room.

She lay in the dark and thought about the book with Agnes's impression in it and what it might contain, what Agnes had thought was worth preserving outside the official record, what she had known that she had kept and kept and kept, waiting for the right reader to find it.

She thought: you kept everything for me.

She thought: I am going to find what you left.

She thought: I am going to finish what you started.

She fell asleep on this, which was the best kind of thought to fall asleep on: the kind that pointed forward rather than back, the kind that had direction in it.

Forward was the right direction.

She had always known that.

She was only now learning to move in it.