Dune
Someone just asked me what works of fiction I found had informed my spirituality. Without hesitation I answered with Dune. It may seem an easy answer to make considering its subject matter, but it is one of the most affecting books (of that vein) I've ever read. It should be read twice - once for the story (as entertainment), and once as a proper religious text, for that's really what it is. I find the bits and pieces of philosophy very sensible, very Zen in many ways. I think I'll go through the series again soon and make notes this time.