Expanded from the novella of the same name.
BEFORE SCIENCE, BEFORE MAGIC, THERE WAS CHAOS
Holger Carlsen was a skeptical engineer with no interest in fairy tales. So it seemed very unfair that a near-miss in a firefight had somehow transported him into a world of knights, dragons, witches and fairy-folk, and even more unfair that he had arrived just as the forces of Chaos were preparing to march forth. Both sides in the looming battle thought that Holger was the key figure in the conflict, able to throw back the powers of Chaos; Holger had no idea how he was supposed to do that.
Arrayed against him were dragons, giants, elven warriors, the beautiful sorceress and seductress Morgan le Fay - indeed all the forces of Chaos. On his side was a vague prophecy, a quarrelsome dwarf, a beautiful woman able to transform herself into a swan - and a magnificent battle-horse and a full set of armor which had been waiting for him when he entered this magical realm. On the shield were three hearts and three lions, a clue to and proof of his true identity - if he could live long enough to learn its meaning...
Poul Anderson (1926–2001) was born in Pennsylvania of Scandinavian stock. He started publishing science fiction in 1947 and became one the great figures in the genre, serving as President of the Science Fiction Writers of America, winning many Hugo and Nebula awards, and also winning the Gandalf (Grand Master) Award.