Over Sea, Under Stone | Susan Cooper

On holiday in Cornwall, Simon, Jane and Barney Drew discover an ancient map in the attic of the Grey House, where they are staying with their mysterious Great-Uncle Merry. They know immediately that it is special. But it is much more than just a map. It is the start of a quest to find a grail, a source of great power that could contain - or resurrect - the powerful, age-old forces of evil in the world. And the Drews are not the only ones searching for it.