What details in the preface of the book, "The Screwtape Letters," allow C.S. Lewis to create verisimilitude?
The entire book is written in the form of letter correspondence. The Preface sets that stage.
The explanation of similar literature and the genesis of some of his concepts in the "Letters" also makes it more of an introduction to a factual observation than a fictional story. His links to his personal experiences and accounts of others also lends an air of "truth" to the subsequent tale.