The Way of Zen by Alan Watts
(I listened to it as an audiobook). It changed the way I think and feel (or at least how I react to feelings). Other interesting books about philosophy/spirituality are
Jonathan Livingston Seagull by Richard Bach
Siddhartha by Hermann Hesse
The Power of Now by Eckhart Tolle
The last one is the least recommended, I'm not sure I got anything out of it, but I read it in in the same period of time as the ones above, so I might have learned something subconsciously.