Belgariad 3: Magician's Gambit by David Eddings
Many thousands of years ago, two prophecies came into being and a moment was fixed, when only one would determine the future. This moment, a clash between the maimed god Torak and the descendant of the Rivan king, is approaching . . . Garion, was brought up as a farm lad but is now beginning to understand the extent of his part in the prophecy, and working hard to control his sorcerous power. He is travelling towards this meeting in a sinister tower where the evil god lies sleeping. With him is the wise sorcerer Belgarath. And Ce'Nedra - a wilful young princess who refuses to believe in sorcery., but is terrified that Garion will die in the coming confrontation . . .