Terry Pratchett is, as always, hilarious. We follow the Wizzard Rincewind as he desperately runs away from another adventure which, as it turns out, is the adventure he's trying to avoid. 
Meanwhile the other Wizards of Unseen University get stuck back in time (while trying to locate Rincewind) on an island with the god of evolution who is just starting to figure things out.  
While this is a top-tier Rincewind-Runs-Away novel, it is, at its core, a book that deals with the myths and stories about the creation of worlds. This is woven in so smoothly with the perspective of the faculty of Unseen University, whose main focus is asking why all the time and not bothering to listen to an answer. So the whys about creation and evolution hang in the air, giving the reader something to think about. 

Bonus: I always enjoy the little parodies and references Pratchett slips into his works. My favorite in this one is "Crocodile Crocodile Dongle" (who I gather to be a parody of Crocodile dundee).