Which C.S. Lewis book presents Christianity through a series of letters from a senior demon?

The C.S. Lewis book presented as letters from a senior demon is The Screwtape Letters. It was first published in book form in 1942 by Geoffrey Bles.

The narrator, Screwtape, is an experienced tempter who advises his inexperienced nephew Wormwood about undermining a human referred to as “the Patient.” Each chapter takes the form of a letter, allowing Lewis to discuss temptation, pride, distraction, prayer, suffering, and Christian belief from an inverted perspective.

Lewis wrote the work as a satirical and imaginative Christian apologetic. The demonic viewpoint is not meant to endorse Screwtape’s advice; readers are expected to recognize the moral errors exposed by the letters. Lewis later found the voice difficult to sustain and did not write a conventional sequel.

The book began as a series of letters in The Guardian newspaper before appearing as a volume. Its epistolary form and ironic viewpoint made it one of Lewis’s most widely read works.