Playing at the World is written by Jon Peterson and is an exhaustive look at the origins of roleplaying games and wargames of all types. What makes this book valuable and useful is the author's reliance on primary sources. Thanks to the development of the internet, and rise of various collector's communities. (like the Acaeum) Jon had a wealth of material to use as the foundation of his research. The result is one of the first (and hopefully not last) serious and impartial look at the origins of our hobby. I am almost done with the first chapter and while written in dry academic tone it is highly readable and at no time I felt I was being drowned in minutiae.
To his credit the author clearly lays out what the book is and isn't. Notably the book doesn't have much in the way of personal stories about the individuals of the early days. He admits there is plenty of material but he felt it would get of the primary purpose of Playing at the World, which is to document and explain the origins of wargames and tabletop roleplaying. He feels with this book future authors will have a framework around to use when writing about other interesting aspects of the early days of the hobby.
Finally this books marks the public release of the mysterious map of the Great Kingdom of the Castles & Crusades Society. Greyhawk and Blackmoor fans will be surprised at what the map really looks like, I know I was.