Optimus Yarnspinner is an aspiring author-lizard from Lindworm Castle, a city of highly literate dinosaurs. His dying uncle leaves him a manuscript -- not his uncle's manuscript (the uncle wrote only one tedious book about cauliflower) -- no, this is the work of some mysterious unknown. It is the perfect ten-page short story. Who reads it laughs, cries, shouts in exaltation, and gives up on aspiring authorship forever, because the perfect story already exists.
Seized by curiosity, Optimus seizes the manuscript and sets out for Bookholm, the city where everything is about books. All the business are bookshops, hotels for people in between bookshops, restaurants for people book-shopping, etc, etc. Everyone is an author or a critic or a bookshop-owner. Book-hunters organize safaris into the crypts to find lost treasure-books. The booby-traps are poison books, the monsters are books with teeth. The lamps are phosphorescent jellyfish (to reduce the risk of fire). You get the idea.
So, this sounds like whatever The Phantom Tollbooth is -- allegorical satire. It's not that. It's not about a child, and it lacks Norton Juster's insightful innocence. Instead, this is a straight-up adventure story: Optimus is kidnapped, dumped into the underground labyrinths of Bookholm, meets literary gnomes, comes face to face (-ish) with the Shadow King, and (of course) finds his muse.
But "adult" fantasy adventure is supposed to stomp along in the mimetically-plausible tracks laid down by Tolkien. World-building! Ecology! Convince the reader that this could really happen! (Presuming the principles of magic, as laid out in Appendix B.)
...I exaggerate. But this sort of semi-allegorical setting really is confined to kid-lit, in my experience. And then, becoming an author is not at all a kid-lit trope. What else could I compare this thing to? The Hero from Otherwhere? Lud-in-the-Mist, for goodness' sake?
Oh, I'm going on about genre boundaries, I don't know why you're still listening. The City of Dreaming Books is a fine little adventure story: creepy in places, triumphant in places, with lots of charming pen-and-ink illustrations by the author. There are many terrible puns. (Bookholmian authors are all anagrams, starting with Aleisha Wimpersleake.) Pixar could make a terrific film out of it (probably skipping the anagrams). It is not the perfect 450-page fantasy novel that will end literature, but it's a good read.