Three Hearts and Three Lions

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

During World War II, Holger 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 used in Three Hearts and Three Lions include:

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.
- The narrator of the prologue and epilogue, to whom Holger tells his story, explicitly says Holger never got that lucky in our world because he was too nice a guy -- too concerned with not hurting girls' feelings to be any good at making advances.
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: With the twist that, unlike in most "Send Me Back" situations, the fantasy world is where he came from originally. As one back-cover blurb put it, "For it was Earth that was his exile...."
Shapeshifting Lover
Shout Out: Just as the Matter of France is true in this fantasy world, so they have legends based on fact and fiction from our own, a Faerie duke referring to "epical chansons about the Emperor Napoleon...." He specifically mentions a Napoleonic officer, Gerard, from several stories by Arthur Conan Doyle.
Taken for Granite: The Giant.