The Grey King — Susan Cooper

There is a legend in North Wales about a harp of gold, hidden within a certain hill, that will be found by a boy and a white dog with silver eyes -- a dog that can see the wind. Will Stanton knew nothing of this when he came to Wales to recuperate from a severe illness. But when he met a strange boy named Bran and Bran's white dog Cafall, he began to remember. For Will was no ordinary boy, but the last-born of the Old Ones, servants of the Light, immortals dedicated to saving the world from domination by the force of evil, the Dark. And it is Will's duty, as he now learns, to wake -- with the sound of the lost golden harp -- the six sleepers who must be roused from their long slumber in the ancient Welsh hills to prepare for the last dreadful battle between the Dark and the Light.