Amazon.com Book Description (ISBN 0575042176, Hardcover)

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.

Amazon.com Book Description (ISBN 0061020672, Mass Market Paperback)

When last seen, the singularly inept wizard Rincewind had fallen off the edge of the world. Now magically, he's turned up again, and this time he's brought the Luggage.

But that's not all....

Once upon a time, there was an eighth son of an eighth son who was, of course, a wizard. As if that wasn't complicated enough, said wizard then had seven sons. And then he had an eighth son -- a wizard squared (that's all the math, really). Who of course, was a source of magic -- a sorcerer.