The Screwtape Letters, by C.S. Lewis, is an epistolary novel containing the letters of a demon, Screwtape, to his nephew Wormwood. Wormwood is a tempter, assigned to lead a human being astray so Satan can take his soul, and he's having some trouble with his charge. Screwtape offers his nephew advice, mostly in the form of explanations of the human mindset and human behavior, and which strategies for leading a human astray might prove effective.
It's quite a clever book, and although we never learn the human's name we come to feel for him and his family. We learn a lot about the demons too, and the whole Infernal bureaucracy set up for capturing human souls. Although Screwtape's letters are exceedingly courteous, and he always signs them "Your affectionate uncle," we do get hints that there is not really love among demons and the relationship between Screwtape and Wormwood is really based on nothing more than self-interest. I also found it striking that the demons don't really understand God and his love for humans, and consider his offering them salvation as a pretext or elaborate deception, though to what purpose they can't comprehend.
We're not meant to take this literally, of course, but Lewis is making a larger point: how easily humans fall into sinful ways, and that evil forces have a part in doing this. But by depending on God, we are able to minimize our sin and receive forgiveness for when we do inevitably err. It's a great book for Christians during Lent, but really it's so well-written that I think even non-Christians would find it interesting, and there's no reason it wouldn't be suitable any time of year.