Firstly, I think there should be some distinction between this book and others in the 'true crime' genre; this one isn't written by journalists, it is written by two experts in the field of forensic psychology research, and it shows.
The thing that most impressed me about the book is probably the last chapter; so many crime books focus on the 'gory' details, but this one ends with a sensitive examination of what happens after the crime, after the trial and after the media spotlight, and explores how those left behind try to pick up the pieces. I think this demonstrates most clearly the sense of humanity that runs through the book; it is so easy from newspaper reports in the mass media to forget that every victim was a real person who was loved and is missed.
The book, understandably given the authors, approaches the topic from an academic standpoint, but this doesn't put non-psychologists at a disadvantage, as the explanations are clear and well thought-out, even when examining some complex issues around the development of offending behaviour. Not only does the book explore issues relating to Robert Napper, but also provides some insight into sexual offenders and murderers in general, which is again accessible to academics and the general public alike.
One small point to make to Amazon, however; I'm not sure, given Professor Alison's defence of Colin Stagg against the heavily criticised sting operation which was led by discredited psychologist Paul Britten, it's such a great idea to be advertising Britten's 'Picking Up the Pieces' book on the same page! For further deconstructions of why this is such a bad idea on Amazon's part, there are a couple of excellent chapters on the Stagg case in Professor Alison's book 'The Forensic Psychologist's Casebook'; this is far more academic in tone to 'Killing in the Shadows', but still written in an accessible way and highly recommended for anyone wishing to broaden their understanding of the case, and of the subject of forensic psychology in general. In particular, the chapter featuring Canter and Alison's deconstruction of Britten's involvement and the way this shaped the police investigation into Stagg manages to be both eloquent and well-argued, without losing the entirely appropriate sense of anger at the way Britten's involvement damaged the profession.
In short, 'Killing in the Shadows' should appeal to anybody with an interest in offender behaviour and police investigations.