The Eyes of the Dragon by Stephen King (used)
A kingdom is in turmoil as the old king dies, murdered by a strange and horrible poison. While the land of Delain mourns, the evil wizard Flagg, hatches an unscrupulous plot, which sees the King's eldest son Peter imprisoned for his father's murder, and the youngest son inherit the throne. Only Peter knows the truth about his own innocence and the evil that is Flagg. Only Peter can save Delain from the horror that Flagg has in store. But first, he must escape from the high tower.