Over Sea, Under Stone by Susan Cooper

This book starts a sequence describing a conflict between the Dark and Light in (more or less) modern England, though its roots stretch back to Arthur's days. In this book, three children go to the Cornish coast for a holiday and discover an old manuscript that claims to describe the hiding place of the grail. The children, with a bit of help from their Great Uncle Merry (apparently Merlin), must find the grail before the forces of the Dark can stop them. The story was entertaining, but I get the sense that this series would have been much more entertaining to me were I a bit younger when I first encountered it.