I'm hard pressed to answer this question (and I suspect I'm not the only one, seeing as it's been so long unanswered) without knowing more precisely what you mean by "tell the truth." A work of literature can be said to have various meanings, some of them mutually exclusive, and few (if any) constraints on viable interpretations. So in that sense, it would tell the truth, because the range of what it "tells" is so very broad. But it would be the same with the visual arts, wouldn't it? If "telling the truth" is understood to be some kind of correspondence with an external state of affairs, then it seems other areas of knowledge (mathematics, for instance) would "tell the truth" better. Aesthetics isn't my area, and so far I'm only dabbling in Phil of Literature; I'm afraid I can't do better than this for an answer. But it is an intriguing start to a conversation. I hope you pursue it.
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.