I intend to re-read this book now, but my hasty thoughts about it from previous readings (just to get the thread going) start back before I first read the book. I remember thinking "Letters from one devil to another! Fire! Brimstone! Blood! Excitement!" So on reading the first letter the thing that struck me was that Screwtape is polite, refined, and quite charming. His wickedness is mundane, his advice practical - it's Evil on a petty scale.
Genius! - because even if at first it seems less important than the grand idea of the war for souls being filled with drama it allows CSL to present his case* couched in terms we can understand quite simply.
[* I'm assuming we all agree that CSL had a case to present - being that God was good and that getting to heaven was something worth the effort…]
Not only this - the reversal of Screwtapes advice (which is what we would be expected to follow to be good Christians) would sound very preachy and picky if laid out as a set of minor "do's and don'ts" - but arguing the other side, playing devils advocate - CSL manages to point out where the smallest slip can lead to the souls ruin without any soapbox moralising. Add to that the fact that Screwtape is a witty devil and writes a very entertaining letter & we can discern what a clever chap CSL was.
On top of this already worthy structure CSL manages to comment on not just matters of the moral soul, but also bring in a more general social satire of the "rich, smart, superficially intellectual, and brightly sceptical" set and finally presents us with a book that is not just about the Christianity but about mankind in general.
The lesson is not just a Christian one - the message is that unkindness leads to petty cruelty which in turn leads to inhumanity. And Satan aside - if mans inhumanity to man isn't the definition of evil then what is?