The subtitle is The Hope for Healing Human Evil, and evil is the topic of the book. But “evil” has a somewhat more restricted meaning here than is usually the case. What Peck means by the word is the attitude of mind and heart of those incredibly self-centered people who are willing to make any sacrifice (of anyone else) and tell any lie to preserve their own self-respect. Because they are such skillful liars they are often well-thought-of, and not largely represented in our prisons.
Peck is a psycho-therapist, and seems to regard that as the ultimate tool for dealing with mental problems. Permit me to discount that a little. He does worry that psychotherapy is apt to be ineffective with the “evil” patient, partly because the patient is essentially never willing to be helped and partly because the doctor must have an incredible love to overcome his natural revulsion.
I reserve judgement on his sort-of descriptions of two exorcisms: he deliberately didn't give enough details. The result isn't very enlightening, nor very germane to the examples of evil people he provides.
He also attempts to address “group evil,” using MyLai as an example, but this really needs a lot more fleshing out. He winds up expanding his penumbra of blame so far that you either have to snicker or envoke Original Sin.
One of his main goals is to try to reconcile religious goals and psychological goals by encouraging a scientific analysis of sin. I don't think he'll win many psychologists over, even with the chapter on exorcisms; and since his theology seems more than a little out of the orthodox mainstream I doubt that he'll win support from Christian leaders either.
BUT. His case histories of the “evil” people are evocative, and you'll recognize people that fill the bill. Unfortunately. Perhaps not so extreme as the father who gave his younger son as a Christmas present the rifle his older brother had killed himself with, but still evil. And for the sake of what you'll learn about them, put up with the rest and read the book.