I can point to several books on my shelf that relate the history of the folk revival and incorporate a broader definition than the 1954 interpretation. Of course, the 1954 definition is open to interpretation as well, it is just that some folks have a more finite set of standards. There is a danger when music is collected of losing the setting and standards that made it "folk" in the first place. Music is not meant to be put on a shelf and looked at.