The Land of the Silver Apples (Sea of Trolls Trilogy, Bk. 2)

Jack is amazed to have caused an earthquake. He is thirteen, after all, and only a bard-in-training. But his sister, Lucy, has been stolen by the Lady of the Lake; stolen a second time in her young life, as he learns to his terror. Caught between belief in the old gods and Christianity (790 AD, Britain), Jack calls upon his ash wood staff to subdue a passel of unruly monks, and, for his daring, ends up in a knucker hole.