Thanks for the heads up on that, I will make a note of that book.That would be based on Robert Ressler's work? Pioneered psychological offender profiling in the 1970s.
One of the officers who interviewed Kemper in real life recounted getting stuck in a cell with him during shift change, and having to bluff Kemper that the prison authorities let FBI keep a defensive weapon long enough to prevent him "twisting his head off".
It's in Ressler's book "Whoever Fights Monsters", which is a cracking read on the topic of the descent into some bad places.
This is what the series is based on.
Meet the FBI agent who inspired the hit Netflix series
Edited @Boumer your chap is the elder of the two characters although there is a big difference in the fictional characters personal life and the real life Ressler.