Discworld by Terry Pratchett. Not one book but dozens (but skip the first two at first) about the discworld that is carried on the backs of four continent-sized elephants that in turn ride on the back of Great A’Tuin, the World Turtle, and he swims through space. There are different loose series of stories about the Witches, the Wizards of Unseen University, the City Watch of Ankh-Morpork, the personification of Death (my favorite) and more. Very funny. Read this article at io9 for a good introduction.