I have read it several times...very good account of such stuff. This book is one of the sources for the chapters focusing on demonic evil in Scott Peck's book, People of the Lie. Another good book on evil.
I have witnessed demonic evil over a person first hand. It is hard to describe it in mere words. "Horrible" and "Terrifying" are a good start.
But, one thing that that most people don't realize is that, after a time, there is a banality of evil. You sort of accept it without being shocked anymore, and I believe that is part of Lucifer's attack. You see that in our culture. Images, words, videos, crimes, and actions that would have shocked us at one time are now ho hum, and elicit no great reaction.
Only God can give us the discernment to see this evil, and only God can provide the strength to overcome it.
"Evil is the exercise of power, the imposing of one's will upon others by overt or covert coercion". "The core of evil is ego-centricity, whereby others are sacrificed rather than the ego of the individual."
That is not evil, only bad. Bad people do things for selfish gain; evil people commit their crimes for the sheer sake of evil. The only ego involved in such acts is found in the demon who owns these twisted souls.
That is one of the reasons that true evil is so disconcerting to liberals. It destroys their central myth that people are good and circumstances drove the poor victim (criminal) to their desparate acts.
I have only the deepest distrust for Peck since he declared that people opposed to abortion were evil, and more generally, conservatives.