Wonderful in some areas, worthless in others. (three stars)
Jon Peterson has written a huge, very ambitious tome. As far as historical data goes, the work is incredibly devoted, including almost-unnecessary minutiae, but there is good reason to include it all, in order to give credit where due and to debunk myths in other places. The author has done a massive amount of work to provide as much information as possible.
However, the minute Peterson starts speaking of the influence of rules on play, narrative, and especially immersion, the fact that he is almost totally ignorant of existing research shows through. I therefore found the book very valuable (5 stars) on some parts, harmfully oblivious on others (barely even one star).
For anything beyond exacting details of the early years, Michael J. Tresca's "The Evolution of Fantasy Role-Playing Games" is the weapon of choice. For the precise history of how and why it all began (and that alone), Peterson's absolutely great.