The Broken Sword by Poul Anderson

Overview: Thor has broken the sword Tyrfing so that it cannot strike at the roots of Yggdrasil, the tree that binds together earth, heaven and hell. But now the mighty sword is needed again to save the elves in their war against the trolls, and only Scafloc, a human child kidnapped and raised by the 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 who has taken his place in the world of men.