The Sea of Trolls (by Nancy Farmer)

Jack angers Frith with an accidental magic spell, and she threatens to sacrifice Lucy unless he undoes it, which he doesn't know how to do. So he goes on a quest to Jotunheim, the land of the trolls, to drink from the Well of Mimir and learn how to undo the spell. Accompanied by Olaf and Thorgil, a girl berserker wannabe, he heads into the land of ice, where dragons, trolls, and the mysterious Norns await.

Although Lucy is an annoying and thoroughly unbelievable character (the author could take a lesson from Suzanne Collins, author of Gregor the Overlander, on how to take small children on an adventure), and the life force, from which Jack draws his power, is a little too Star Wars, readers will be having too much fun to worry about the details.