The Great Gatsby question

Hi,

I am trying to remember the name of a book with the following details:

Basic plot is a well known professor gives a lecture and then is invited to a man's house. The man is not the nicest guy in the world, and he has a son who is developmentally challenged. While the professor is getting ready for bed, the boy gives him a story he has written that tells of a knight (or something of the sort) who has a meeting with the Devil, who he is afraid of. I thought the name of the book was "Peter's Story" or "Freddy's Story" or something of the sort but haven't been able to track it down.

Thanks,

Ken