Which C.S. Lewis book is framed as correspondence from a senior demon to a junior demon?

The Screwtape Letters is framed as correspondence from a senior demon to a junior demon.

C.S. Lewis’s The Screwtape Letters consists of letters from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. The letters discuss how to tempt and mislead a human known as “the Patient.” Lewis uses the inverted viewpoint to create a satirical examination of human weakness and spiritual self-deception.

The book was first published in book form in 1942 after appearing in serial form in the Anglican newspaper The Guardian. Its epistolary structure is often mistaken for a conventional supernatural novel, but it is primarily a work of Christian apologetics and moral satire. Lewis dedicated it to his friend J.R.R. Tolkien.