Godslayer
Torn away from the midst of a firefight in Vietnam and catapulted to a time and place where the Norse gods fight a deadly war of their own, Al Larson finds himself locked into an elvish body on a world where swords and spells are the means of battle.
Al must adapt swiftly---or die.
For the gods have marked him as their own private battleground, and Al's only chance rests in completing the quest Freyr has set him, a quest that will lead him to the very gates of Hel, where he must save a god---or destroy one!
(Extracted from the jacket.)
Found out more about the World of the Bifrost Guardians.