In his day, his best selling work. Perhaps outdone by the Narnia books since then. But still a sharp edge picture.
It has an introduction where he talks about his beliefs of angels and devils, and how his are symbols at that. And then we get into the letters from Screwtape to his nephew Wormwood, about the business of temptation. And the sharp-eyed sketches of human life and insight into human character that are its chief attraction.
There are characters, there is even something like a plot, but what really attracts is the discussions of such things as laughter, erotic love, and pacifism vs. patriotism.