Which C.S. Lewis book presents imaginary letters from a demon advising a junior tempter?

The C.S. Lewis book presenting imaginary letters from a demon to a junior tempter is The Screwtape Letters. It was published in book form in 1942.

The senior demon Screwtape writes to his inexperienced nephew, Wormwood, about how to tempt a human being. Lewis uses the reversed perspective to examine moral weakness, distraction, pride, fear, habit, and self-deception. The demons treat evil as their professional work, creating the book's distinctive satirical tone.

The work first appeared as a series of letters in the Anglican newspaper The Guardian in 1941. Lewis later collected the pieces into a book. The format lets him discuss religious and ethical questions indirectly, because the reader sees human behavior through the distorted reasoning of Hell's bureaucracy.

The Screwtape Letters is sometimes confused with The Great Divorce, another Lewis work about spiritual realities. The two books differ in form: The Screwtape Letters is an epistolary satire, while The Great Divorce is a philosophical fantasy framed as a journey.