The Club Dumas by Arturo Pérez-Reverte. Very good if you're into books in general as it is a book about books. Polanski's "The Ninth Gate" is based on this.
One of my favorite movies. How does the book elaborate on the movie?
Perez-Reverte is one of my favorite authors. Polanski's film is a faithfulish adaptation of half of the book, but, in "club dumas", the whole "9th gate" story is embedded in a broader investigation around Alexandre Dumas' books, so, it's almost as if Polanski had adapted a subplot of the novel (okay, let's say "the" subplot, not "a" subplot). This makes a very different tone, as the book is much more ironic towards conspiracies, and has a string "Foucault's Pendulum" vibe all the long.
Also, Depp is extraordinarily well cast, he corresponds perfectly to the novel's character's description. However, as always, Seigner is horribly miscast : the Adler girl in the novel is actually very charming.