Sourcery

The Discworld is very much like our own - if our own were to consist of a flat planet balanced on the back of four elephants which stand on the back of a giant turtle, that is…
All this books and stuff, that isn't what it should all be about. What we need is real wizardry.
Once there was an eighth son of an eighth son, a wizard squared, a source of magic. A Sourcerer.
Unseen University, the most magical establishment on the Discworld, has finally got its wish: the emergence of a wizard more powerful than they've ever seen. You'd think the smartest men on the Disc would have been a little more careful what they wished for.
As the drastic consequences of sourcery begin to unfold, one wizard holds the solution in his cowardly, incompetent hands. Rincewind must take the University's most precious artefact, the very embodiment of magic itself, and deliver it halfway across the disc to safety... If he doesn't make it, the death of all wizardry is at hand.
And the end of the world, depending who you listen to.
The Discworld novels can be read in any order but Sourcery is the third book in the Wizards series.