Do you think _Zen and the Art of Motorcycle Maintenance_ is categorically a philosophy book, or because it's a novel, it cannot be in that classification?
There is no reason that a novel cannot be a work of philosophy; in fact, I would argue that many novels are exactly that. Philosophy broadly construed is "the love (study/seeking/etc.) of wisdom," which can certainly be pursued through fiction. A little more narrowly, a work in philosophy would employ a certain style of inquiry, methodical or systematic or logical. Even more narrowly, it would contain references to, or even excerpts from, the standard philosophical repertoire; Jostein Gaarder's Sophie's World comes to mind here. While I have never read Zen and the Art of Motorcycle Maintenance myself, from what I have heard about it, I would consider it a work in philosophy broadly speaking -- that is, it gives you the impetus to ask the kinds of questions traditionally associated with philosophy.