The Sea of Trolls

Jack was eleven when the berserkers loomed out of the fog and nabbed him. It seems that things are stirring across the water, the Bard had warned. Ships are being built, swords are being forged.
Is that bad? Jack had asked, for his Saxon village had never before seen berserkers.
Of course. People don't make ships and swords unless they intend to use them.
The year is A.D. 793. In the next months, Jack and his little sister, Lucy, are enslaved by Olaf One-Brow and his fierce young shipmate, Thorgil. With a crow named Bold Heart for mysterious company, they are swept up into an adventure-quest that follows in the spirit of The Lord of the Rings.
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.