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

The C.S. Lewis book presenting correspondence between a senior and junior demon is The Screwtape Letters. Lewis published it in book form in 1942.

The senior demon, Screwtape, writes letters to his inexperienced nephew Wormwood, who has been assigned to tempt a human patient. The letters describe human habits, distractions, pride, fear, and self-deception from the demons’ inverted point of view.

This unusual structure allows Lewis to discuss Christian moral and spiritual ideas indirectly. Readers must recognize that the demons’ advice is deliberately distorted: what they call success is generally presented as failure from a Christian perspective.

The work began as pieces published in the Anglican newspaper The Guardian. Lewis later collected them into a book, adding a preface and an additional letter. Its epistolary form and satirical voice have made it one of his most widely read works outside the Narnia series.