The Picture of Dorian Gray. Best book I have ever read, I finished it a couple of days ago. Explores art, beauty, human nature, the "soul", philosophy. Has the best characters that I have ever seen (read). Lord Henry Wotton kept me on the edge of my seat whenever he went on one of his little speeches about pleasure and youth and beauty.
As a side-note, I decided to start watching the film. Turned it off after 20 minutes, it's dreadful. Nothing like the book, didn't capture what it was really about. I know I only watched 20 minutes so it's not a proper judgement, but I didn't want to spoil my memory of the story with it.