Three Hearts and Three Lions

Three Hearts And Three Lions is a fantasy novel by Poul Anderson.

During World War II, Holgar is in Denmark, part of La Résistance. Except that at one crucial moment, he finds himself transported to a fantasy world, where the Matter of France from Chivalric Romance, Charlemagne and his paladins, is true. And that he has some sort of connection with this world that he can not remember.

A major influence on Dungeons and Dragons.

Tropes found in this work

 * All Trolls Are Different: Three Hearts and Three Lions is, if not the source of the regenerating troll concept, at least one of the earliest known examples.
 * Animated Armor: Guarding the passes into fairyland.
 * Chick Magnet: Holger, to his surprise. Apparently he never got that lucky back home.
 * Door Step Baby: Holger again.
 * The Fair Folk: Complete with the allergy to cold iron and holy names.
 * Human Sacrifice
 * Kill It With Fire
 * Knight in Shining Armor
 * The Paladin: The paladins of this work are where the original inspiration for the Paladin class from Dungeons and Dragons came from.
 * Send Me Back
 * Shapeshifting Lover
 * Taken for Granite: The Giant.