Over Sea, Under Stone by Susan Cooper

Highly recommended!!!  Great for kids and adults, a fascinating read!

The first volume of Susan Cooper's brilliant and absorbing fantasy series, The Dark Is Rising.

On holiday in Cornwall, the three Drew children discover an ancient map in the attic of the house that they are staying in. They know immediately that it is special. It is even more than that -- the key to finding a grail, a source of power to fight the forces of evil known as the Dark. And in searching for it themselves, the Drews put their very lives in peril.