For anyone interested in the psychology of serial killers I would HIGHLY recommend The Last Victim by Jason Moss.
It's not an academic/educational book, it's a true story of a young man who befriends John Wayne Gacy (The Clown Killer) whilst he's behind bars. It is fascinating, well written and gives a chilling insight into the mind of a very dangerous serial killer. It contains snippets from letters they exchanged and detailed accounts of their encounters.
I couldn't put it down!