One book that you could read for the rest of your life and enjoy everytime, because you can get something new out of it with each and every reading. I suppose this can be taken as a "which book would bring with you if you were stuck on a desert island for the rest of your life?" question, of sorts. Go!

Lolita by Vladimir Nabokov, because I've never read a novel where there was such dimension to the writing and the narrator/main character. Lolita makes me feel like I'm reading the book from inside Humbert Humbert's brain.