The Grey King by Susan Cooper

In this book Will Staunton gets to go on his own quest (with less help from Merriman Lyon and the Drew children) for one of the artifacts (a golden harp). He goes to Wales, the author gives a good try at indicating proper Welsh pronunciations, and confronts the Grey King, who is trying to stop him from retrieving the harp and waking the sleepers. The plot in this book was more skillfully managed than in other books in this series. The Arthurian connections are becoming more clear as well.