A Walk in Wolf Wood is one of Mary Stewart's young adult books, and is much more evocative than The Prince and the Pilgrim. The historical setting is wonderful, really giving an idea of what life in 14th century Germany was like.
Two children, a brother and a sister, on a family picnick follow a sad-looking man into the dark and forboding Grimm-like woods. They soon find themselves involved in an Adventure featuring a werewolf, and evil sorceror, and a plot to over throw the king.
This is a kid's book. It's short, it's simply written. Some people have complained that it's too simply written, and that it panders to children. I read it in the tub at the age of 23 and enjoyed it greatly.