Here are a couple words defined before you start to read, that will help you to understand the letter better.
Incongruities: not compatible, not harmonious
Flippancy: Excessive silliness and lack of steadiness especially regarding grave or sacred matters.
This letter discusses the patient's increasing circle of friends. Screwtape comments that much laughter can be heard and goes on to explain the 4 causes of laughter.
What are the 4 causes of laughter?
Which 2 causes are NOT useful to the devils?
Why do the devils hate fun?
Why is flippancy great for the devils? How can they use it to their advantage?