Which C.S. Lewis book presents an imagined conversation between a senior and junior demon?

The C.S. Lewis book presenting an imagined conversation between a senior and junior demon is The Screwtape Letters.

The book is structured as a series of letters from Screwtape, an experienced demon, to his nephew Wormwood. Screwtape offers advice about tempting a human patient, while Lewis uses the demonic viewpoint to satirize weaknesses in human behavior and criticize spiritual complacency.

The Screwtape Letters first appeared as a newspaper serial in the Anglican newspaper The Guardian in 1941 and was published in book form later that year. Lewis dedicated the work to J.R.R. Tolkien, though the two authors differed in literary style and religious approach.

The conceit can cause confusion because the book is not a conventional narrative about demons. Its letters work indirectly: readers must infer the virtues Lewis is defending from the temptations Screwtape recommends. The work became one of Lewis’s most popular Christian writings.