Which 1942 C.S. Lewis book is narrated through letters from a senior demon?

The 1942 C.S. Lewis book narrated through letters from a senior demon is The Screwtape Letters.

The book consists of fictional correspondence from Screwtape, an experienced demon, to his nephew Wormwood. Screwtape advises Wormwood on how to tempt and mislead a human being known as “the Patient.” Lewis uses the demonic viewpoint to create satire about human weaknesses and spiritual self-deception.

The work was first published in book form in 1942 by Geoffrey Bles. It had previously appeared in serialized form in the Anglican newspaper The Guardian. The letters are not presented as a conventional plot-driven novel; their structure is a sequence of advice, observations, and tactical instructions.

The book is often confused with Lewis’s other works of Christian apologetics, but its method is distinctive. Instead of speaking directly as a theologian, Lewis reverses the perspective and lets a demon reveal the habits that can damage a person’s moral and spiritual life. That ironic approach gives the book its memorable tone.