Over Sea, Under Stone by Susan Cooper
Series: The Dark is Rising #1
Three children on vacation find themselves on a treasure hunt that is much more than it seems. They don’t even realize how much depends on them and their only aid is a mysterious great uncle. They are not the only ones searching though and the competition is fierce. The story is consuming. Children will be at once intertwined with the youthful characters and adults will have their childlike curiosity brought back to life. The historical and legendary references make it interesting to everyone.