This is tough. I'm an avid reader and have gotten different things from different books. Some of the highlights:

Books of fucked-up lives: Exposure, The Saskiad, Deerskin. Puts my own life into perspective, lets me live vicariously and understand the psychology of fellow outsiders, even fictional ones, a little better.

Books that bring me to tears: The Martian Chronicles, Slaughterhouse-5, The Time Traveller's Wife. The greatest themes of all: the futility of slaughter, the helplessness of love, the frustration of desire.

Books that caught me by surprise: Vurt is bizarre and twisted and wonderful, with an ending that satisfies even as it defies expectation. The Princess Bride, far better than the movie - don't believe me? Read it. Cryptonomicon makes math and history topics of fascination and delight.

The one book I turn to again and again, the one book that's as comforting as a blanket and hot tea, the one book I've read since childhood and will read until I can no longer turn the pages: The Outlaws of Sherwood. It is an unorthodox retelling of the Robin Hood legend, and I am forever discovering new nuances and subtle turns of phrase that render it a work of beauty. It is not high literature, far from it, and it has its flaws, and it is so close to my heart that I cannot separate the two.