The Doom Stone (The Zone Unknown) By Paul Zindel

Jackson is always psyched to visit his aunt Sarah when she's working on an anthropological dig. This time she's in England, at Stonehenge, and Jackson can't wait to see the massive and mysterious stone formations in person. But then he witnesses a vicious attack on a young man, and another on his beloved aunt Sarah. A savage beast no one has ever seen before is on the prowl. Now it's up to Jackson and his new friend, Alma, a gravedigger's daughter, to stop the beast. All the clues lead back to Stonehenge, where he and Alma must risk their lives to solve the mystery of the monster stalking the countryside-before it's too late. "Even reluctant readers won't be able to put this one down."-Boo