The Sea of Trolls

The year is A.D. 793; Jack and his sister have been kidnapped by Vikings and taken to the court of Ivar the Boneless and his terrifying half-troll wife; but things get even worse when Jack finds himself on a dangerous quest to find the magical Mimir's Well in a far-off land, with his sister's life forfeit if he fails.
Other threats include a willful mother Dragon, a giant spider, and a troll-boar with a surprising personality -- to say nothing of Ivar the Boneless and his wife, Queen Frith, a shape-shifting half-troll, and several eight foot tall, orange-haired, full-time trolls. But in stories by award-winner Nancy Farmer, appearances do deceive. She has never told a richer, funnier tale, nor offered more timeless encouragement to young seekers than "Just say no to pillaging."