Sounds pretty crazy right? But it's real. By interviewing real captured serial killers these guys were able to learn how to profile active ones during ongoing investigations. They got so good at it that they could call out little quirks in their personalities like if they had a stutter or a troubled relationship with their mother. Essentially they had to learn how people like this think because there weren't eye witnesses or anything like that. And most SK's are very intelligent.
All of it is real too. The book is written by John Douglas, who helped found the 1st forensic profiling unit in the FBI. Edmund Kemper is one of the main serial killers he interviewed to help start all of this.