The Revenge of the Wizard's Ghost — John Bellairs

Thirteen-year-old Johnny Dixon, possessed by the ghost of the evil Warren Windrow, is dying. And it seems as if there is nothing anyone can do. In a desperate effort to save Johnny's life, his two best friends, Professor Childermass and Fergie, race to the Windrow estate, hoping by some miracle to find something -- anything -- they can use to free him. There, as demonic forces surround and threaten them, they are caught in a most horrifying fight for their own survival.