Which C.S. Lewis book presents a fictional correspondence between a senior demon and his trainee?

The C.S. Lewis book presenting correspondence between a senior demon and his trainee is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters consists of letters from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. The letters offer advice about tempting a human being and preventing that person from developing a relationship with God.

Lewis uses the inverted viewpoint to expose habits such as distraction, vanity, resentment, and self-deception. The book is not written as a conventional narrative with a straightforward human protagonist; its argument emerges through the demonic advice and the irony created by the narrator’s perspective.

The work began as a series of pieces associated with wartime broadcasting and was later collected as a book. Its epistolary structure distinguishes it from Lewis’s more direct apologetic works, including Mere Christianity, and from the allegorical journey described in The Great Divorce.