The Chronicles of Narnia: The Silver Chair by C.S. Lewis

The Silver Chair follows the point of view Jillian Pole, a school acquaintance of Eustace Scrubb. She travels to Narnia with Eustace, and Aslan gives her directions on how to find Caspian's lost son, Prince Rilian. Jill and Eustace, along with Puddleglum the Marshwiggle, travel across the land in the search for the lost prince. They encounter giants who eat humans, cave people, and a witch, who has captured and brainwashed Prince Rilian. After coming to his senses and killing the witch, the group travels back up to the surface to reunite Rilian with his father.