The Screwtape Letters is a fictional series of letters in which a senior devil, Screwtape, instructs his nephew, Wormwood, in how to tempt a man known only as "the patient". Despite the fact that we never see any of Wormwood's letters, Lewis expertly has Screwtape refer frequently to Wormwood's letters, therefore enlightening us to the general content of the junior devil's reports.
While it is easy to read and understand, the reader must always keep in mind that since the letters are written from a devil's perspective, the opinions, motives, etc. portrayed as God's (referred to as "the Enemy") opinions, motives, etc. are not always -- rarely, actually, -- God's true opinions, motives, etc., but are rather twisted as a devil would perceive them to be.
Throughout this book, Lewis uses these letters to point out tactics that Satan uses against both Christians and non-Christians; most of these tactics are subtle, and not ploys one would immediately associate with devil's work or even recognize as a device to draw people away from God. The subtly of these tactics only make them more dangerous, as the longer people are unaware of such distractions weaving their way into the background of their lives, the more time Satan has to tug us away from God and His will.
I'd recommend this book to anyone and everyone. The writing and dialogue is easy to read and understand, while simultaneously bringing to our attention the wiles of the Devil.