I read this as devotional literature, but it also works quite well as a satire or just a generally comical novel. Lewis' wit is on full display here, as Screwtape chides Wormwood for each failure and attempts to give him advice on how to proceed. My copy of the book has been misplaced, but the tactics that Screwtape advises Wormwood to use will be familiar to any Christian, and, while reading through, there were several times when it was nice to think, “Oh, so I'm not the only person who has a problem with that.”
Wormwood ultimately fails in his task to distract the young unnamed Christian, who (SPOILER) dies fighting World War II. The last section of the book is Lewis' “sequel” to Letters, Screwtape Proposes a Toast. It's more of a lengthy essay than a novella, ubt it addresses several aspects that had come to light since the publication of the initial novel. I know this review doesn't do it justice, but Screwtape Letters is one of the best books I've read this year, and I'd highly recommend it. Also, John Cleese reads the audiobook. Just saying.