'Zen Mind, Beginner's Mind' by Shunryu Suzuki.

It's one of the first book I've read about zen (the first was '101 Zen Stories', which I recommend as well). Didn't know it was a classic when I bought it fifteen years ago. I've reread it many, many times.

Edit: I forgot 'Branching Streams Flow in the Darkness' by the same author.