Five years have passed since Jack the apprentice bard and Thorgil the shieldmaiden first made their appearance in Nancy Farmer's "The Sea of Trolls," a thoroughly entertaining adventure set in eighth century Britain. The young Saxon boy and the feisty North girl went on a quest to the very source of life, the tree Yggdrassil, to save Jack's sister Lucy. The two unlikely friends were back two years ago for "The Land of the Silver Apples," the middle book in Farmer's trilogy.