The Grey King - Susan Cooper

There is a Welsh legend 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 recover from a severe illness. But when he met Bran, a strange boy who owned a white dog, he began to remember. For Will is the last-born of the Old Ones, immortals dedicated to saving the world from the forces of evil, the Dark. And it is Will's task to wake-with the golden harp -- the six who must be roused from their long slumber in the Welsh hills to prepare for the last battle between the Dark and the Light.