I just finished this excellent book by C. S. Lewis. It was a re-read as I have read it a couple times before, but always get something new out of it. Although humorous in a way, it is a thinker book and takes a bit of brain power to understand what he is talking about.
The book is written from the perspective of a high-powered demon. Therefore "The Enemy" is God and "patients" are human beings that the demons are desperately trying to win over to "Our Father" (Satan) in subtle ways. Apparently, Mr. Lewis did not enjoy writing this book as he had to look deeply into the spiritual forces alive in this world and how swayed humans can be without even realizing it. The way the book is written, as a demon looking for chinks in a particular Christian's armor, it shows us some of our vulnerabilities and helps us begin to see just how open we can be to the attacks from the true enemy! Attitudes, patterns and easy-way-outs can be temptations from the enemy that we used to just accept as matter of fact. Even the way the demon tries to use current events and close relationships to his "patient" to get him to turn away from the Lord and trust in himself show many tactics that the enemy might use in our own lives as well.
It took me awhile to read this book... probably because I read at night before going to bed and its a hard one to read when you are exhausted, but I highly recommend it.
There is also a short sequel of sorts to the "Letters" called, "Screwtape proposes a toast". You can read it online here. In this concise essay, Lewis brings to light some of the evils that can be hidden in the name of "Democracy". VERY enlightening after our recent election. Anyway, enough on that... just read it if you have a few minutes.