A good setup: correspondance between supernatural creatures. In this case it's demons from hell discussing how to damn human beings. Twain did something similar in Letters from the Earth, and played it for laughs with great success. C.S. Lewis takes a more serious tack, and in my opinion overplays it. Like many converts to Christianity, he's just a little too heavy-handed about his faith. The premise is so rich for parody, the nonstop sermonizing is a letdown. I don't have any special criticisms of the content, or even the writing... if the book had been half as long, I'd probably have given it five stars.