In the early days of D&D, when actual copies of the rules were hard to come by, the game was passed around as oral tradition. Most of that wild era is lost now (as is the custom with oral traditions), but now unearthed is this 1974 D&D approximation, written by a 14-year old who thought the game was created by a friend of a friend, and who was trying to codify what he must've learned through playing, like someone painstakingly enumerating all the rules to your playground games of foursquare.
The book is a marvel, and I can't recommend it too highly, either from the perspective of the evolution of RPGs or as a document of "folk" culture.