1973 novel by Mary Stewart. Second book of The Merlin Trilogy, a retelling of the Arthurian myth (see Arthurian Mythology and The Chronicles of King Arthur) from the enchanter Merlin's perspective.
This sequel to The Crystal Cave begins where the former left off, as told by Merlin (Myrddin), "prince and prophet." Gorlois, Duke of Cornwall, has just been slain in battle, leaving Uther Pendragon, High King of Britain, free to claim the widow Ygraine (Igraine) as his Queen. With Merlin's help, Uther seduced Ygraine the night of Gorlois' death, and she is pregnant with his son—the future King Arthur. Due to the scandalous circumstances surrounding his conception, Arthur is taken away from his parents at birth to be raised by Duke Ector of Galava as a foster brother to Cei (Sir Kay).
The Hollow Hills deals indirectly with Arthur's early boyhood (Merlin travels during this time, but keeps track of the prince's progress in visions and correspondence), his education by Merlin, and the events that led to his ascension to the throne of Britain. Through Merlin, Stewart explains the genesis of the legends surrounding Arthur's kinghood, even while relating events as they actually happened in her interpretation of the classic myth.
The story continues in The Last Enchantment and a fourth novel, The Wicked Day, which is told from the perspective of Arthur's bastard son Mordred. As in The Crystal Cave, the author provides a listing of her sources, historical and literary, and a summary of the legend which inspired her writing. Good stuff.