I loved that book of Robert Pirsig, but to say that it is a philosophical book? No, I would not. It is a beautiful travel and the narrator is struggling with his attitude towards life having the feeling to experience something beautiful and very worthwile.
I would call it a grasping-life book or a gateway to philosophy book. It makes you ready to start reading real philosophers. Enjoyed every moment reading it and it inspired me to start reading the books of the philosophers themselves.
Right, they're supposed to die before their books sell. But Robert Pirsig--the author of the book--is still alive! I imagine that Simone De Beauvoir, Jean Paul Sartre, Jacques Derrida, and Michel Foucault were wealthy philosophers. Noam Chomsky must be pretty wealthy as well. Ludwig Wittgenstein was wealthy because of his family, but they say he gave up the fortune. Bertrand Russell I believe was wealthy?