The Walker (Briony Williams) Unabridged edition

Twin boys run away from their au pair on a trip to the Oxford countryside. They race into some woods where they see a man chopping. At first they think he is chopping wood, but what he is holding in his hand is not an axe, and the shape on the ground is not a tree. Their au pair, Sylvie, sets off to look for them. Briony Williams has moved to Oxford. The story begins as she speeds across the city to the murder site, where Sylvie Bec has been found dead - killed by a single blow that crushed her skull. The forensic pathologist says he has never seen anything like it...