Books like The Broken Sword

Thor broke the sword Tyrfing to save the roots of Yggdrasil, the tree that binds earth, heaven and hell. Now the elves need the weapon for their war against the trolls. Only Scafloc, a human kidnapped and raised by elves, can hope to persuade Bolverk the ice-giant to make Tyrfing whole again. But Scafloc must also confront his shadow self, Valgard, the changeling in his place among men.