Lila
Follow up to Zen and the Art of Motorcycle Maintenance, and where that book asks what Quality is, this one tries to build an entire metaphysics on top of the answer. Phaedrus takes a boat down the Hudson with a woman called Lila who is visibly coming apart, and the question sitting underneath the whole thing is whether she has any Quality at all. Around that Pirsig stacks anthropology, a division of everything into inorganic, biological, social and intellectual levels, and an argument that morality is just what happens when those levels collide.
I’m glad I read this one rather than the motorcycle book even if it’s considerably less loved. I often attribute a (hugely positive) turning point in my marriage to having read the opening chapters of this book.