The Light Fantastic by Terry Pratchett

It is known as the Discworld. It is a flat planet, supported on the backs of four elephants, who in turn stand on the back of the great turtle A’Tuin as it swims majestically through space. And it is quite possibly the funniest place in all of creation… As it moves towards a seemingly inevitable collision with a malevolent red star, the Discworld has only one possible saviour. Unfortunately, this happens to be the singularly inept and cowardly wizard called Rincewind, who was last seen falling off the edge of the world.