This is an interesting read for lovers of early blues.
It is not about musical details - you won't find out about Robert Johnson's guitar tunings, or who influenced Muddy Waters. Rather, it discusses the people who collected the blues via recordings (e.g. the Lomax family) and those early collectors who decided what should be regarded as good music.
The author is concerned with the motives of early enthusiasts, who filtered the music for us. For example, some sought out music that was uninfluenced by radio or gramaphone, some were politically motivated, looking for music of the people, and some looked for great art in the blues.
Sometimes, the book is rather dryly academic in style, but don't let that put you off. Anyone interested in early blues and how is was selected will find it a worthwhile read.