I read this book in paperback several years ago after the Roman Polanski treatment of it in The Ninth Gate came out. I found a hardcover edition in Santa Fe last year and picked it up, and decided to reread it a few months ago. As usual, the main thing that impresses me about Perez-Reverte books is how good the translation is - it's never awkward as, for example even the best translations of Verne from French to English are. Maybe Spanish just translates better, but P-R's prose is really rich and sharp in English - he's one of the best writers I know in terms of his language and ability to paint a picture, which is saying a lot for a translation.
His talent for crafting characters and situations is one of the highlights of the book, as well as the dusty rare book book world that's just fun to read about. The chills that the book's detours into demonic manuscripts create are also a highlight. The actual plot is not much - just being along for the ride is the reason for reading it. I enjoyed it, although my recollection is that the movie (which I think discards most of the Dumas elements) is actually a better tale.