The detective motif
These stories begin with motifs that point the reader toward a genre, specifically detective fiction, like Sherlock Holmes and the many other classics in that genre. The motif tells the reader that the author himself appreciates detective fiction (demonstrating his knowledge of the craft by employing typical devices), and it also underscores the Trilogy's quest for truth and meaning.
The Don Quixote allusion
In City of Glass, the protagonist spends much of his thought life wondering about art. He is a writer turned detective who is now immensely troubled by his life as a PI—not what he was hoping for when he was writing. The allusion to Don Quixote adds a layer of desire, because he understands himself to be like Cide Hamete Benengeli who watches Don Quixote's escapades from the sidelines.
The replaced person
In the opening story, we see a writer turned detective. In the last story, we see a detective fiction author stealing a story from a missing person, and instead of solving the case, he replaces the person in their family. This is a symbolic response to the question of City of Glass because it admits through satire that the author secretly wanted to escape himself by inventing stories and characters to "replace" consciously.
Paul Auster as a symbol
As a meta-narrative element, Paul Auster the author writes himself into the story, both as a writer and as a detective. This means that although the reader might have been supposing the writer himself intended to be the narrator, actually, he has created a thinly veiled other person, eventually named Daniel Quinn. His inclusion as a character in the book represents the difference between the artist and the narrator.
The motif of insanity
This novel explores all the ways that writing and detective work (which are thematically linked in each of the three stories) lead characters to insanity. This happens largely because the protagonists are emotionally challenged by their quest for truth, and in the meantime, there are psychic shortcuts that help them get to conclusions faster. The main one is escapism, but every one of the stories shows escapism leading to insanity. Daniel Quinn describes his own insanity in the prose. Blue feels crazy by his quest to understand Black, in "Ghosts," and in "The Locked Room," the protagonist abandons his identity completely, literally replacing someone else.
