The Hollow Hills, published in 1973, tells the story of Merlin's travels and Arthur's boyhood and early years of Kingship.
Once again, the characters are alive and vibrant, the history and places are characters of their own.
Merlin becomes a father figure to Arthur, a mentor. He teaches him nothing of magic, but of statesmanship and politics, of warfare. We are also introduced to Guenevere, to Bedivere (Lancelot's parallel... Lancelot was a late French addition), and to most of the rest of Arthur's court. Merlin, whom I enjoy much more than Arthur, is strongly present throughout the book, unlike The Last Enchantment, or most all, The Wicked Day.