I came across this old thread on Books that changed your life and thought it was amazing that nobody mentioned The Lord of the Rings by J.R.R. Tolkien. When I was 14 those three volumes hit me like a ton of bricks and its fantasy world has stayed with me ever since.
A few others:
Tao Te Ching by Lao Tzu … the classic of the Tao
Buddhism for Dummies … read this very much later but it was a very good intro
Falling into Grace by Adyashanti … the clearest writing on non-denominational foundations to the search for enlightenment