Terry Pratchett: Sourcery

Description

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.
Unseen University has finally got what it wished for: the most powerful wizard on the disc. Which, unfortunately, could mean that the death of all wizardry is at hand. And that the world is going to end, depending on whom you listen to. Unless ofcourse one inept wizard can take the University's most precious artefact, the very embodiment of magic itself, and deliver it halfway across the disc to safety...