This is a wonderful fantasy series. Simon, Jane and Barney are enlisted by their mysterious great-uncle to help fight the forces of evil, the Dark. Will Stanton, who appears to be an ordinary child, but is the last-born of the immortal Old Ones and therefore, not really an ordinary child at all, is also joining forces with them. They fight fear and death in the darkly brooding Welsh hills, in a quest through time and space that touches the most ancient myths of the British Isles.
A beautiful three part series - this is book 2.
There is a tradition that within a hill in North Wales a golden harp will be found by a boy followed by a white dog with silver eyes. Will stanton, the sast of the Old Ones, knows nothing of this until he comes to Wales to recover from an illness. There he meets Bran, aboy with white hair and golden eyes whose dog Cafall has white hair and silver eyes. Memory awakes in him and he sets forth on the quest for the harp with which to continue the battle between the Dark and the Light.
The Grey King is the fourth in the Dark is Rising series.