The Grey King by Susan Cooper is the fourth book in "The Dark is Rising" sequence. Young Will is sent off to Wales to rest and recover after a severe illness. He wanders over the sheep farm struggling to make his mind remember what he has forgotten in his illness until he meets Bran and his dog Cafall. Then his memory returns, and he realizes again that he is one of the Old Ones and that it is not mere chance that has brought him to Wales. It is Will's quest to find the harp that will awaken the six sleepers before the coming of the dark for the last final battle. This time he must tackle the quest without Merriman and apply all of the learning he has acquired. He's not completely alone for Bran and Cafall become part of quest, although for Bran it is a quest not for just a golden harp but for the secret to his own identity.
Another excellent book in this series that I would recommend for ages 11 and up. There is plenty of action, suspense, and mystery woven into long-standing Celtic and English myths and legends to propel the reader along on this latest quest. Excellent read!