The Sea of Trolls by Nancy Farmer

Main Character: Eleven-year-old Jack, a Bard's apprentice
Location: England, Scandinavia
Time period: 8th century
Genre: YA Fiction, Fantasy, Norse Mythology

Jack is honored to be chosen as the Bard's apprentice, but his training has barely begun when danger comes to their village. The Northmen are coming, sent by Queen Frith--a half-troll who has a grudge against the Bard, who she calls Dragon Tongue. When a Nightmare, sent by Frith, robs the Bard of his mind, it is up to Jack to protect the village. But Jack's magic fails and he is taken captive by the Northmen, along with his little sister Lucy.

Thus begins an heroic quest which will lead Jack not only to the Northman's lands, but to the land of the Trolls and even to the Great Tree of Life, Yggdrassil. On his quest, he is aided by Olaf, a giant berseker; Thorgill, a shield maiden; and Bold Heart, a crow with unusual intelligence.

The Sea of Trolls takes place in a time when the old pagan beliefs still survive but Christianity is taking hold in England. Magic and monks co-exist (not always comfortably) side by side, though great changes are coming.