Earthquake Weather by Tim Powers
A young woman possessed by a ghost has slain the Fisher King of the West, Scott Crane. Now, temporarily freed from that malevolent spirit, she seeks to restore the King to life.
But Crane's body has been taken to the magically protected home of Pete and Angelica Sullivan, and their adopted son, Koot Hoomie. Kootie is destined to be the next Fisher King, but he is only 13 years old -- too young, his mother thinks, to perform the rituals to assume the Kingship.
But not too young, perhaps, to assist in reuniting Scott Crane's body and spirit, and restoring him to life . . .