Alan Watts. He writes a lot of non-fiction about Buddhism, but with an approach to it from a western perspective. It changed how I think, how I approach my days, and how I live, and as weird as it sounds I even started shaving my head after getting into his books and Buddhism as a whole. Some books that I started with were "The Book", "The Wisdom of Insecurity", and "The Way of Zen". What started it all though, was when I stumbled across this audio of him speaking, which was animated by the South Park guys... Oddly enough this one little youtube link lead me down a very constructive path of self discovery and renewal.
Also, Kurt Vonnegut has been integral to my life and how I think about life, reality, and society as a whole. Anything written by him for the most part has had some lasting effect on me. But if I had to name one, it would be Sirens of Titan by him.