Written in the form of letters, The Screwtape Letters by C. S. Lewis is an amusing and insightful correspondence between a senior devil, Screwtape, and his obstreperous and incompetent nephew, Wormwood, a "young fiend." All of the letters are from Screwtape to Wormwood, and the subject of the correspondence is a human being, newly converted to Christianity, whom Screwtape refers to as "the patient." Throughout the correspondence, Screwtape tries to help Wormwood tempt the patient away from Heaven and into Hell. He encourages his nephew's successes, suggests various and devious ways to enter the man's thoughts and influence him, and berates Wormwood for his failures as the man begins attending church, goes through various ups and downs of faith, falls in love with a Christian girl, and is called to service in the War. All in all, the letters and "Screwtape Proposes a Toast," the last chapter of the book, are a witty and cautionary commentary on the state of the modern human soul.

At first, Uncle Screwtape is paternal with his nephew Wormwood and quite indulgent. He gives him lots of excellent advice about how to subtly tempt a human being away from thoughts of God and one's neighbors toward thoughts of self, materialism, doubt, cowardice, and denial of the existence of God. All this is done so the soul of the human being will end up with "Our Father Below." Screwtape is a demon par excellence—very experienced, very subtle, and very wise (wise as a serpent, as a matter of fact). The believing reader will recognize just how insightful into human weaknesses Screwtape is. Indeed, C. S. Lewis mentioned he had only to look so far as his own soul and the goings on within it to be able to write the doings and thoughts of Screwtape.

Wormwood, Screwtape's nephew, is less than a stellar student when it comes to influencing a human being in a hellish direction. He makes many blunders. Screwtape's letters to his nephew grow increasingly impatient, angry, and unforgiving as the patient's faith and sincerity grow. The man becomes engaged to a Christian girl, entering her charitable and loving circle of family and friends, and grows in his faith. He also participates bravely in a war. He is triumphant in both the physical and spiritual wars in which he engages. Screwtape lambastes Wormwood with sarcasm and malice for his bumbling. Since Hell is a predatory place, Wormwood's failure will result in Screwtape consuming him.

The reader gets a further glimpse into Screwtape's (twisted) mind when he makes some highly astute commentary on the general situation of society and deviltry in modern life. This commentary takes place in "Screwtape Proposes a Toast," the last section of the book. Screwtape is the guest of honor at the annual dinner of the Tempters' Training College for young devils. Both witty and cautionary, Screwtape rises to summarize modern humankind's situation and Hell's promising prospects for winning the war against God by capturing most of God's children. He claims Hell will be populated not so much with grandiose sinners as with ordinary people who, through conformity, materialism, worldliness, muddy thinking, and petty corruption, lose all sight of Heaven in their lives.

Two interesting notes appear in the beginning section. The first is that The Screwtape Letters is dedicated to the author of The Lord of the Rings, the great J. R. R. Tolkien. Tolkien and C. S. Lewis were close personal friends as well as fellow faculty members at Oxford. A second interesting note is that in the front matter C. S. Lewis quotes two famous Christians, one Protestant and one Catholic: Martin Luther, the founder of Protestantism, and Thomas More, a Catholic saint. Both are quoted as saying that the best way to deal with the devil is to laugh at him. Lewis took this advice to heart in writing his rollicking and rueful The Screwtape Letters.