In The Screwtape Letters C. S. Lewis takes a unique perspective on human nature: what do humans look like from the perspective of devils? Lewis begins with a disclaimer that while he does believe in the symbolic forces represented by devils, he does not consider them essential to his Christian faith. The reader need not believe that winged, horned creatures inhabit the earth in order to appreciate The Screwtape Letters. This book is about human nature not Christian cosmology.

In form, the book is a series of letters from a veteran devil, Screwtape, to his young devil nephew, Wormwood, who has just embarked to his first attempt to tempt a human soul. Wormwood has been assigned a young Englishman living in England during the Second World War to lure away from "the Enemy," i.e. God. In each letter Screwtape responds to his nephew's previous letter with words of advice on how to most effectively keep Wormwood's "patient" away from God. For example in Screwtape's fifth letter to Wormwood, he chastises his nephew for becoming "delirious with joy" from the horrors of the world war while he neglects "the real business of undermining faith and preventing the formation of virtues."

The real power of The Screwtape Letters comes from Lewis' subtle insights into human nature which often leapt off the page and hit home with me. Lewis's devils do not run around causing evil in an obvious and crude ways like starting natural disasters or spreading fatal diseases. Instead they look to exploit common human weaknesses and then allow the humans to unwittingly cause all the evil in the world. To this end Screwtape advises Wormwood that he corrupt his patient's prayers for his mother by making sure that "they are always very 'spiritual,' that he is always concerned with the state of her soul and never with her rheumatism." Screwtape explains that, "since [the young man's] ideas about her soul will be very crude and often erroneous, he will, in some degree, be praying for an imaginary person, and it will be your task to make that imaginary person daily less and less like the real mother—the sharp-tongued old lady at the breakfast table. In time, you may get the cleavage so wide that no thought or feeling from his prayers for the imagined mother will ever flow over into his treatment of the real one." It is subtle insights like this one, woven throughout The Screwtape Letters, that make it such a valuable book for anyone on a spiritual path.