Discworld Book 5: Sourcery HC

There was an eighth son of an eighth son. He was, quite naturally, a wizard. And there it should have ended. However (for reasons we'd better not go into), he had seven sons. And then he had an eighth son ... a wizard squared ... a source of magic ... a Sourcerer. Sourcery sees the return of Rincewind and the Luggage as the Discworld faces its greatest - and funniest - challenge yet.