Breaking Open the Head by Daniel Pinchbeck is one of my all time favorite books. I've read it several times and never fail to come back to it. It is addictive and every time I read it again, I always find something new. The way the main character adapts to life and its surroundings to the way things balance each other out is very interesting. Quite a trippy journey.
FYI, the cover keeps on changing. Mine was the first edition I guess.