City of Glass is about language failing to hold the world in place. Peter Stillman senior's experiment to recover Adam's tongue, Quinn's identity dissolving across three names, and a case that begins with a wrong number all run the same argument: identity here is built out of language and nothing else.
Language
Every theme in City of Glass runs back to one failure: language has stopped doing the work it once did. Peter Stillman senior locked his son in a room for nine years, permitting no contact with the outside world and beating the boy for any sound he made, babble or word alike. His theory: a mind raised with no language taught to it would produce, on its own, the tongue Adam spoke in Eden, where a word and the thing it names are the same object. Nine years produced no such tongue. They produced a son who at twenty-five speaks in pieces and loses the end of a sentence before he reaches it.
Stillman senior sets out his reasoning to Quinn directly, using a broken umbrella as his case study. Once an object stops doing what its name promises, he argues, the word for it is false, and a new word is owed. He proposes to build that vocabulary himself, word by word. By the time Quinn meets him, his own sentences have come apart into what he describes: lists, asides, clauses that stop before they resolve. The theory and the man's speech state the same fact twice. The characters page covers what else drives him.
Identity and the double
Quinn writes detective novels as William Wilson, a name borrowed from Poe, and thinks through a private detective named Max Work, invented as the books' hero. He answers to neither. When a caller asks for the Paul Auster Detective Agency, Quinn corrects him twice, then on the third call says he is Paul Auster and takes the case. Nothing forces the choice. He makes it because the name is offered and no one is present to argue him out of it.
The case runs on a second doubling Quinn cannot resolve by watching: the client is Peter Stillman, and the man he is hired to follow is also named Peter Stillman, father and son sharing one name across the plot's whole length. Quinn meets a man at Grand Central claiming to be Stillman senior and the book gives him no way to settle which man is the impostor, if either is.
By the account's end, Quinn has moved through three names and stopped being able to return to the first one. The novel makes its claim about identity through that arithmetic rather than stating it outright: a name holds only as long as somebody keeps answering to it. Quinn stops answering, and the record of him stops with him. The ending page covers what the last chapters do with that fact.
Chance
The case begins with a wrong number. Quinn's telephone rings twice after midnight on separate nights, and the caller wants an agency that does not exist, run by a detective who shares Quinn's address but not his name. Nothing about the call is aimed at Quinn in particular. It is a misdial, and the entire plot follows from his decision not to correct it a third time.
Chance keeps this pressure through the rest of the book. Stillman senior's walking routes, plotted on Quinn's map, trace shapes that spell letters across Manhattan, a pattern Quinn builds a theory around and cannot confirm belongs to anything beyond his own reading of it. The writer named Paul Auster, once Quinn finds him, turns out to be working on an essay about who wrote Don Quixote, a subject with no connection to the case beyond the name the two men happen to share.
The wrong number is not an isolated device. Chance is Auster's most consistent subject, and the rest of his fiction returns to entry points a character does not choose and cannot decline once they have arrived.
The red notebook
Quinn buys a red notebook before he begins following Stillman senior and fills it with addresses, times and the shapes of Stillman's routes across the city. It works as his casebook, the tool his own detective novels rely on, except nothing written in it resolves into a solution. Once the case has nowhere left to go, Quinn keeps filling the notebook anyway, alone in the empty room that had been the Stillmans' apartment, until the pages run out.
The notebook is also the reason the reader has any of this. An unnamed narrator, introduced only in the last pages, states that the notebook was the only material he had to build the account from, and that his report stops where the notebook stops rather than where Quinn's story does. Nothing in City of Glass is reported that did not first pass through those pages. Who that narrator is, and what he leaves unresolved, has its own page.
Auster returned to the red notebook as an object across decades of fiction and memoir. The recurrence is tracked in full on the motifs page, alongside the other objects and situations he kept rebuilding book to book.
The locked room
The term names two separate things in this book, and they are worth keeping apart. The first is literal: the room where Stillman senior kept his son from infancy to age nine, without light, without speech directed at him, and with a beating for any sound the boy made. That room is where the language experiment happened and where it produced the son the book presents.
The second is the room Quinn ends in: the empty apartment across from the Stillmans' old building, where he watches, writes and runs out of food and pages both. The book does not call this a locked room, and no one locks Quinn inside it. He stays by choice, and the choice is what closes the door.
In detective fiction generally, a locked room is a killing committed inside a space sealed from outside access, the puzzle being how anyone reached the victim at all. City of Glass borrows the phrase's shape rather than its plot—a sealed space, a mystery that will not resolve, no way in that explains what happened once. The phrase becomes the title of The Locked Room, published by Sun & Moon Press in 1986 as the third novel of The New York Trilogy. That book is not a sequel to this one and does not reuse its plot. It reuses the shape.
Postmodernism and metafiction
City of Glass is postmodern by a specific mechanism, not by mood: a character named Paul Auster appears inside the novel, married, with a young son, at work on an essay about who actually wrote Don Quixote. He is not the book's author. He is a device the book uses to make authorship a plot point rather than a fact readers bring from the cover. Quinn, mistaking this character for a detective, spends the novel acting on an error the reader is positioned to share and then asked to notice.
That is the argument for calling the book postmodern, and it is arguable rather than settled. Critics reading it as anti-detective fiction—Alison Russell among them, in a 1990 essay in Critique—describe the trilogy as taking the detective genre's machinery of clues, surveillance and solution and running it until the machinery breaks rather than resolves. Russell's reading treats the case Quinn works as a stand-in for the act of reading itself, one that never reaches a solution because a text, unlike a crime, does not have one to reach.
Whether that makes the book metafiction, anti-detective fiction, or both at once is a live disagreement in the criticism, not a gap in the novel's design. The postmodernism page sets that argument out in full, across Auster's other books as well as this one.
New York as a place
The city in City of Glass is named, mapped and walked block by block, and it is also where the plot's logic comes from. Quinn's surveillance depends on Manhattan's grid: he can trail Stillman senior only because the streets are regular enough to plot on paper, and the walks only resolve into possible letters because a grid is a surface a pattern can be read against.
Quinn's relationship to the city is settled before the case starts. Walking is already how he empties himself out, and the case gives that habit a job to do. New York functions in the book less as a setting than as the surface the story needs for its experiment: exact enough to map, large enough to lose a man in.