Over Sea, Under Stone is by Susan Cooper. The first book in the Dark is Rising series. Jane, Simon, and Barney Drew head out on a vacation with their parents to a seaside village in Cornwall. Their "Uncle" Merry has rented the Grey House while the owner, an old sea captain, is away. While rummaging through the house, they find a hidden entrance to the attic and discover an ancient text that shows the way to an ancient grail with mystical powers. They soon discover there are others seeking the grail to use it for evil and they must be stopped at all costs. The book starts out slowly, as it takes a while to establish the setting, but once it starts moving it is hard to put down. Boys and girls will enjoy this series.
Ratings: 5th grade - 8 out of 10.