The narrator turns out to be a friend of the writer Paul Auster, the character inside the novel. He wrote City of Glass from Quinn's red notebook, which Auster passed to him once the case ended. The text states that it does not know what became of Quinn after the notebook ran out. No death or return is confirmed.
Who the narrator is
City of Glass is told about Quinn in the third person until two pages from the end, where an unnamed "I" steps forward. That voice says he is a friend of the character called Paul Auster, the writer Quinn is mistaken for earlier in the book. Auster gave him Quinn's red notebook once the case was over, and the narrator says he built the story from what the notebook held. Nothing before that page signals this narrator's presence. Until then the narration has no speaker at all. The change comes on the last page, without warning, and stays unexplained.
This is the fact readers searching for the ending usually want first, and it settles a common misreading: the narrator is not Quinn, and he is not Paul Auster the author of the book in your hands. He is a minor character, introduced late, reporting at second hand.
The red notebook
Quinn buys the notebook before he starts following Stillman senior, and he fills it with addresses, times and the shapes traced by Stillman's walks across Manhattan. It functions as his casebook, the tool his detective novels rely on, except that nothing in it resolves into a solution. Later, alone in the empty room that had been the Stillmans' apartment, Quinn keeps writing in it until the pages run out.
The narrator states that the notebook is the only material he had to work from. That is why the account stops where the notebook stops rather than where Quinn's story stops. Auster returned to the object across his fiction and his memoir writing for decades after 1985, and the pattern has a page in Auster Studies.
What happens to Quinn
What happens to him next can be stated in a few exact steps. Quinn keeps watch on the Stillman building from an alley across the street for months, eating and sleeping as little as the watch allows, until his money runs out. He telephones Virginia, gets a busy line, goes to the apartment and finds it empty. He telephones Auster and learns that Stillman senior jumped from the Brooklyn Bridge two and a half months earlier. He returns to his own apartment and finds someone else living in it. He then moves into the empty Stillman apartment, writes in the red notebook until the pages run out, and the account stops at that point. The book does not describe him leaving the room, and it does not describe his death.
What the text does and does not say
The narrator states outright, in his voice, that he does not know what became of Quinn once the notebook was full. That single sentence is the novel's clearest answer.
Three things the text supports: Quinn stopped writing when the notebook ran out, the room was later found empty, and no one connected to the case saw him again. What it does not support is a specific fate. Critics have argued for a death in the room, for a walk out into the city and out of the record, and for a reading in which "Quinn" as a fixed identity had already come apart before the notebook ran out. Auster gives material for the first argument and the third, and he declines to choose between them. Treat any confident claim about Quinn dying or surviving as the reader's inference, not the book's statement.
Why Auster ends it there
The open ending matches the rest of the novel's method. Quinn takes on an identity that was never his, works a case built on no evidence, and follows a language experiment that fails on its terms. Stopping at the edge of the notebook rather than at the edge of the plot keeps the book's central claim in place: identity, on these pages, is built out of language and nothing else, notebook included, and once the writing stops there is nothing further the narrator can honestly report.
Readers who want this shape worked through twice more can follow it into Ghosts and The Locked Room, the second and third books of The New York Trilogy, which collected all three in 1987. For the plot from start to finish, chapter by chapter, see the chapter guide. For the rest of the cast, see the characters page.