Do you view C.S. Lewis’ The Screwtape Letters as a standalone work? A Pilgrim in Narnia has discovered something that may indicate Lewis intended his collection of demonic letters to be part of a larger fictional context.
The Screwtape Letters begins with a letter of its own. “I have no intention of explaining how the correspondence which I now offer to the public fell into my hands. There are two equal and opposite errors into which our race can fall about the devils.” Your copy of this work attributes this letter to Lewis himself, but one early edition attributes it to someone else, a character from another of Lewis’s works. Brenton Dickieson spells out more details here.