Sourcery by Terry Pratchett

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.