If you've the perseverence and objectivity to penetrate Peck's occasionally rather annoying 'evangelism', you'll be rewarded with a fascinating insight into the behaviour of 'evil' people and their impact on others.
The People of The Lie is without doubt an important attempt to understand and describe 'scientifically' a very common though as yet, not formally classified personality/character disorder. However, Peck's emphasis on the 'supernatural' dimension, particularly his observations on the value of exorcism is inappropriate. It obscures some extremely worthwhile evidence in support of his contention, that there is scope to define formally a 'new' disorder to help psychiatrists and psychologists manage both 'evil' people and their victims. I suspect this book is not universally popular among these professionals.
Nevertheless, its worth the effort. Whether you're a believer, agnostic or athiest, The People of the Lie offers much food for thought. So far as I know, people who are just plain bad are not well catered for by formal psychology theory; bad people really do exist, they're not simply damaged people who do bad things so perhaps Peck's book is a worthwhile attempt to expose such people for the benefit, principally of their victims.
I was a victim and as such the book has helped me greatly. As a pychologist, I would have preferred a less 'emotional' and more accessible approach to what is a seriously under-researched phenomenon.