Three Hearts and Three Lions
Poul Anderson
 Holger Carlsen is an Allied covert operative who assists the Danish Resistance to the Nazis. After an explosion, he finds himself carried to a parallel universe, which proves to have the Matter of France as its historical past. There he finds that the evil of Faerie is encroaching on humanity. His quest finally leads him to discover that he is Ogier the Dane, sent to this universe by Morgan le Fay, and to fight the battle that drives back the evil. This also thrusts him back into our world, in which he is able to ensure that Nazis can not stop a crucial escape from occupied Europe. At the end of the novel, he is seeking his way back in the other world, where he had fallen in love with a swan may.