The Tao Te Ching (Dao de Jing) by Lao Tse (Tze), although the translations can be very different. I like
this translation.
Journey to Ixtlan by Carlos Castaneda. It isn't his first book, but it is the first I reccomend reading. If you like it, go back to
The Teachings of Don Juan: A Yaqui Way of Life and then on the others.
The Screwtape Letters by C.S. Lewis. While intended as an allegory on Christianity, I would suggest it to non-christians as well, as it makes many points about human nature which transcend the Christian doctrine. Not only that, but I think C.S. Lewis really saw through a lot of the fluff and stuff of modern Christianity. So, before you rip on the religion, maybe read this book. Then, make fun of the Church all you want, but see how it is not truly representative of the religion.
Any of the works of:
Friedrich Nietzsche
Wittgenstein
W.V. Quine
St. Thomas Aquinas
There's a start, I will let you know if I think of more.