Showing Over Sea, Under Stone by 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.