The Light Fantastic — Terry Pratchett

RINCEWIND -- was the most truly inept wizard the flat earth of Discworld had ever known. He was a wizard with only one spell, a spell he would never dare to say, one of the Eight Great Spells froth the magical Octavo. So powerful was this magic that every ordinary useful spell refused to stay in the wizard's mind for even one instant.

Then the Red Star appeared in the sky--and everyone finally understood what the Eight Spells were really for...to ward off the menace of this starry devastation. But the Octavo only had seven spells left. And suddenly the whole planet- from wizards and warriors to druids and demons-were after Rincewind. Yet even if they found him in time, would this inept magician be able to get the only Spell of his life right? Would this be the end of the only honest-to-gosh -flat earth--or some crazy new beginning...?