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.
That’s certainly interesting, but I don’t think it’s as important as Dickieson thinks it is. Or important in a different way. It would appear that Lewis originally wanted Screwtape to be part of the Ransom universe. Then he made an artistic decision not to do that. Perhaps he thought (as I am inclined to) that the two works didn’t really fit together very well.
I wondered if it was a publisher’s decision, that someone said it would restrict or confuse readers to appeal to knowledge of another book in the preface, as if this was a sequel. But it makes sense that Lewis would have made this decision, because if he hadn’t, perhaps we would have heard about it in interviews or letters in the following years.