What is the theme in Mindhunter: Inside the FBI's Elite Serial Crime Unit by John E. Douglas?

A study on prison is a theme in this story. When Douglas arrives at BSU the agency is still under the direction of J. Edgar Hoover and his preconceived notions of FBI training and indoctrination. During the Hoover regime profiling is discouraged to avoid risking FBI embarrassment from an incorrect profile. Behavioral training is done on a "road school" program with veteran agents.