"Zen and the Art of Motorcycle Maintenance" by Robert Pirsig. While it's also a tale of travel and self-exploration (the rediscovery of identity after a nervous breakdown and shock therapy), mostly the book is about thinking. I was a different person after identifying with the narrator who also lived in the mind, and asked questions about basic concepts like 'quality', or qualia of events, objects, roles, and the subjective/objective values they embody or we impart on them. A watershed book for me.