I loved The Screwtape Letters by C.S. Lewis. Oddly enough, I had never read anything by him
This book is considered a classic and was quite entertaining, and very thought provoking. The book is a series of letters written to "Wormwood" by his uncle "Screwtape" during WWII. Screwtape was a highly placed official in the service of "Our Father Below". His letters to his nephew were an attempt to train his nephew in how to bring the nephew's client to their side.
It's a brilliant piece of work!