That is an excellent book!
When it first came out and after I read it, I purchased more books to give away. Everyone was fascinated by it too. It's eye-opening for sure.
I've finished it now, and have to say I liked the first half better than the last half because in the later chapters he gets into demon possession and I have reservations about some of what he said.
Also, his psychoanalytical approach to psychology was by definition Freudian and much of Freud's theories that Peck refers to have been discarded/discounted by psychologists today and a couple things he said in reference to the Oedipus complex and also his therapy methods, what he did or would do, were disturbing. It was at that chapter that I started pulling back.
Also, and this may not seem like much, he said in the preface of the book that he used a neuter pronoun for Satan. Oddly, he also used a neuter pronoun for children:
"Treated badly by its parents, a child will usually assume that it is bad."
Overall, I have very mixed thoughts.