What is the title of C.S. Lewis’s only novel written entirely as letters?

The Screwtape Letters is C.S. Lewis’s only novel written entirely as letters.

Published in book form in 1942, it consists of correspondence from Screwtape, a senior demon, to his inexperienced nephew Wormwood. The letters advise Wormwood on tempting a human subject, while the demonic viewpoint exposes habits such as vanity, distraction, resentment, and self-deception.

Lewis used the epistolary form to reverse the usual moral perspective: readers see ordinary human behavior analyzed by demons who regard spiritual failure as success. The book is satirical rather than a literal guide to demonology, and its imagined bureaucracy gives the argument a distinctive comic voice.

The work first appeared serially in the Anglican newspaper The Guardian. Lewis later stopped contributing to that publication after its editor died. A Grief Observed is a diary-like meditation on bereavement, while The Great Divorce is a fantasy narrative, so neither fits the question’s letter-only form.