The Screwtape Letters by C S Lewis.
It's about Wormwood, a demon being trained in the art of tempting humans and leading their souls to hell by his uncle, Screwtape.
To quote Wikipedia, "C.S. Lewis provides a series of lessons in the importance of taking a deliberate role in living out Christian faith by portraying a typical human life, with all its temptations and failings, as seen from the demon/devil's viewpoint"
It's a neat book. Read it if ya haven't.