The letters of Screwtape, a senior demon, to his nephew Wormwood, which writings are full of the best ways to undermine the faith of "the patient" - the human under Wormwood's care.
Morality
As the tale is written from the perspective of a demon, of course the laws of morality are either turned upside-down or not touched at all. Screwtape encourages his nephew to make "the patient" act in immoral ways in order to destroy him.
Spiritual Content
The story is based on God, Scripture, and spiritual beings. Screwtape and Wormwood are both demons working against God by attempting to destroy His people. Angels are mentioned as enemies of the demons; Satan is called "Our Dark Father" and God is "The Enemy". Also, early on in the letters "the patient" becomes a Christian.
Violence
The Great War (World War I) is mentioned.
Drug and Alcohol Content
Maybe one or two mentions of drinking.
Sexual Content
Screwtape urges his nephew to encourage "the patient" in sexual thoughts, but nothing is graphic.
Crude or Profane Language or Content
When Screwtape becomes annoyed with his nephew, as he often does, he will insult him. However, these insults are never bad.
Conclusion
'The Screwtape Letters' is one of Lewis' most acclaimed writings, and for good reason. The letters show with magnificence the subtlety of Satan in his attack on Mankind, and how spiritual beings are ever at work in the world, though we can't see them. The letters explore many common human sins that the reader can relate to and learn from. This book has limitless values for anyone who wants to learn more about their walk with God.