By my evaluation The Screwtape Letters (1942) is one of CS Lewis's very best books; and the follow up 'essay' Screwtape Proposes a Toast (1959) isn't as good in a literary sense; however, it is very important and ought not to be missed.
In particular, Toast is very astute in understanding the modern (post World War II) demonic strategy - or, to put it differently, Toast tells us what the Global Conspiracy is conspiring to achieve.
I have often said that 'the conspiracy theorists' are broadly correct in diagnosing that an evil cabal lies behind the strategic direction of the modern world. Where they are wrong is that the cabal is ultimately demonic, not human; and consequently its aims are spiritual not material.
In particular, most conspiracy theorists focus on the idea that human suffering and death are being engineered by a cabal of selfish, greedy and perverted humans. But the reality is that behind these humans (who do indeed exist) and controlling them, and dictating the long term direction of evil, there are supernatural evil beings whose purpose is human damnation - not human suffering and death.
The most obvious way that this shows itself is that it is very easy for powerful people to cause suffering and death; and if that was their ultimate goal, then there would be a great deal more human suffering and death everywhere there has been over the past several decades. In fact, the human population has grown by about four billion since WWII, infant mortality has reduced, everywhere, life expectancy has increased everywhere, and there are large areas of the planet that have not suffered war for many decades, or longer.
Clearly, the powers of evil either aren't aiming ultimately at suffering and death... or else they aren't really evil, or aren't really powerful; which I reject on these grounds. What they are aiming at, and what they have done very, very effectively; is clearly laid out in Screwtape Proposes a Toast.
The powers of evil are corrupting, not killing humanity; they are perverting not causing here and now suffering; they are working by pleasure, confusion and distraction rather than pain and focused fear.
The forces of evil have learned from the Second World War, which - in spiritual terms - significantly backfired against evil in the West and triggered a powerful spiritual and Christian revival - of which The Screwtape Letters was itself a product and a trigger.
To get clearer about this - read Toast...