The writing duo of Fletcher Pratt and L. Sprague de Camp were, according to Gary Gygax, great influences on Dungeons & Dragons, particularly in the form of the Harold Shea "Enchanter" series. In my opinion, the collaborations of these two authors were generally better than their solo works. I suspect it's because each author reined in the worst aspects of the other when working in concert. A good case in point is 1942's Land of Unreason, a terrific story about a contemporary American who finds himself transported to the land of Faerie.
The book's protagonist is Fred Barber, a diplomat living in Yorkshire, England during World War II. On the night of Midsummer's Eve, when Barber's hosts leave out a bowl of milk as an offering to the fairies, he decides to make light of the custom by swapping the milk for scotch whiskey. As a consequence of his jest, the fairies who come for the milk become intoxicated -- and more than a little perturbed at his actions. They kidnap Barber, spiriting him off to Faerie, where he's taken to the court of King Oberon to answer for his deed.
Oberon offers Barber a chance to return to his own world if he will first atone for his crime by undertaking a mission on behalf of the fairies. He's to go off into the Kobold Hills -- the source of many magic weapons -- and determine if an ancient enemy of the fairies has returned. Barber reluctantly agrees and sets off through the bizarre landscape of Faerie on his mission. While doing so, he meets all manner of equally bizarre characters and his interactions with them, not to mention the quest itself, set the stage for revelations about the nature of Barber's own existence.
Land of Unreason is a fun book. Its depiction of Faerie is one I particularly enjoy, for this otherworldly land functions according to its own weird logic, one that is largely alien -- and often inimical -- to visitors from our reality, like Barber. Its inhabitants are, by turns, helpful, seductive, and terrifying. One gets the very real sense that mortal men were not meant to dwell in Faerie, something I much prefer to the dewy-eyed romanticism one often sees associated with fairies. I'll also admit that I'm a sucker for tales of modern men transported to fantasy realms, a trope that was once a staple of the genre but now seems to be less common (though it hasn't disappeared entirely). It's well worth a look if you've never had the chance to do so.