What's this book????

Question

It's a series of letters from a demon to his deon nephew about how to tempt a man to "fall"?

Answer 1

The Screwtape Letters by C.S. Lewis

Answer 2

I went to google and typed in Demon letters how to tempt man. Surprisingly, I got exactly what you described. It's called The Screwtape Letters, by C.S. Lewis. There's also a short sequel called Screwtape proposes a Toast. They're both supposed to be a form of christian satire.