Hogfather, by Terry Pratchett
Pratchett's wonderfully funny and whimsical Discworld novels always have a bit of philosophy in them, but I found this book to be especially deep. When the beings who run the universe decide to get rid of human life, their first move is to do away with the Hogfather (the Discworld's version of Santa Claus). Why? Because belief, Pratchett postulates, is what makes us human. As children, we have to learn to believe in the little things, such as the Tooth Fairy, before we can believe in bigger concepts like justice and mercy. These are not absolutes; they are concepts, ideas, impermanent and fragile. Once again, Terry Pratchett's humor and wisdom have impressed me mightily.