The sea of trolls - Nancy Farmer

Jack is an apprentice bard and just beginning to learn the secrets of his mysterious master, when he and his little sister are captured by Viking chief, Olaf One-Brow, and taken to the court of Ivar the Boneless. Ivar is married to a half-troll named Frith, an evil and unpredictable queen with a strange power over her husband's court. Jack is sent on to the kingdom of the trolls, where he has to find the magical well and undo the charm he has cast on Frith. He is accompanied by Thorgill, a shield maiden, aged 12, who wants to be a berserker when she grows up. Together, they are set for a magical and exciting adventure.