I enjoyed reading this book - but I must say, in a case of "style over content", it was because it was a grippingly enjoyable, easy-to read potboiler of an investigation, not because I thought it was a particularly good investigation.
The book falls roughly into two parts - the first being a collection of both historical and contemporary MIB experiences, and how this history and that of some early experiencers played a part in what became a much more fleshed-out modern phenomenon. I had some difficulties with parts of this. While the history behind the first recorded MIB experiences was very interesting and informative, I found some of the the subsequent "cases" a bit hard to swallow. That's no disrespect to the experiencers - all experiences in my book being personal and valid as an experience. But they mostly revolved around exactly the sort of people who already believed in such things and were perhaps more likely to experience unusual phenomena, and the experiences themselves were often breathlessly-related tales relating not much more than the sighting of a man in a suit in a car, or hanging around near where somebody lived, or a temporary problem with a 'phone line. All of these cases could be explained by coincidences or misinterpretation of regular people going about their business - it's only Redfern's colourful use of Lovecraftian adjectives to describe these 'events' that makes some of them worth reading about. And to post photographic 'evidence' such as a photo taken of a man dressed as a MIB at a UFO conference of all places ... that stretched even my patience a bit.
The second part is, in my opinion, in many ways more interesting and important. This section deals with various theories on where the phenomenon may stem from and what it actually is and means. Theoretical discussions take in the 'daemonic' (from Tulpa to Trickster), plain hoaxes and government agents, to hallucinations, time-travellers, and misidentification. There's also a member of the clergy who not too surprisingly supports the theory that anything paranormal and occult is the work of the devil.
I'm with Redfern in concluding that the MIB phenomenon is likely to be a mixture of many different things, from government agents to hoaxers to people with overactive imaginations interpreting a someone in a suit parking nearby as the sinister MIB phenomenon.
So my two questions would be, why devote so much of the first part of the book to experiences that read exactly like such misidentifications? And the second is - if people who poke their noses in to MIB business often end up harassed, hounded, lose their minds or end up dead (leading Redfern, somewhat MIB-like himself, to warn readers of the consequences of dabbling too deeply), how come he chose to?