MEMNOCH THE DEVIL by Anne Rice.
If you want your mind, your brain and your head taken on a metaphysical trip, read this. Good stuff. Makes you really think, plus it's super-entertaining. I've read this at least five times -- it follows perfectly The Vampire Lestat, Queen of the Damned and Tale of the Body Thief, the three Lestat books before Memnoch. I suggest you read them in order -- I didn't the first time but they are all great books on their own. Hope you find a book you like!