Mark Twain's book A Connecticut Yankee in King Arthur's Court comes up several times across the run of John Landis' one hundred percent perfect horror classic An American Werewolf in London, repeatedly underlining David's fish-out-of-water (excuse me... werewolf out of water?) status. His nurse and romantic interest Alex reads it to him as he convalceses; specifically this passage:
"A Word of Explanation. It was in Warwick Castle that I came across the curious stranger whom I am going to talk about. He attracted me by three things: his candid simplicity, his marvelous familiarity with ancient armor, and the restfulness of his company - for he did all the talking. We fell together as modest people will in the tail of the herd...."
And then later, while he's holed up in her apartment as the full moon appears, it is this book that he's picked back up to pass the time with when he has his little, you know, conversion...
In the book the titular Connecticut Yankee idealizes the myths of Arthurian Legend but comes to see it wasn't quite like he imagined, and similarly David references several werewolf movies over the course of An American Werewolf in London - I love how his references are to American movies while Alex, who's a Londoner, first thinks of Oliver Reed when she thinks of a wolf man - in order to color in and understand his situation. There's no need to go to the library and read up on lycanthropes when you've got Claude Rains and Lon Chaney Jr. around to help.