I first read this book in a psychopathology class; it has always stayed in my mind. Everyone should read it. The author's defining characteristics of the psychopath are still as relevant today as they were over 50 years ago: the psychopath (today usually called an "anti-social personality") is much like a gourd; there's an outside (the "mask of sanity"), but inside there's nothing there. No conscience, no guilt, no remorse. They're great actors, though. In their more disturbed forms they're the serial killers, such as Ted Bundy. Cleckley suspected these people were literally monsters, lacking whatever made them human. A must-read for those involved in law enforcement.