I actually really enjoyed this book. So why only two stars?
To me, it read more like a series of interesting (and very well written) Wikipedia articles strung together, rather than a cohesive book. The author introduced many interesting ideas and anecdotes, but I found it severely lacking in any analysis or interpretation. I can easily access facts and information online for free - when I buy a book I'm looking for something more, some insight or fresh knowledge I couldn't get anywhere else. I was also let down by the way the author failed to really define the terms he was using, frequently using 'madness', 'psychopathy' and 'mental illness' rather interchangeably. One of the most fascinating aspects of psychopathy is the debate over whether it is an illness, a personality disorder, a personality trait etc., but this was was only touched upon rather than truly explored.
Overall it's a very interesting read, and the author's tone is particularly engaging, but there's nothing here you couldn't find with a quick web search.