Sourcery. Corgi Audio Books 1994 ISBN 0-552-14011-2

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 has 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.