Samuel Charters - The Bluesmen: The Story and the Music of the men who made the Blues

Charters focuses much more on analyzing the music & lyrics than discussing social conditions. When he does talk about social conditions, he's pretty matter of fact.
Note that many of the recordings Charters mentions have been posted on YouTube by various people. I found it very helpful to listen along while reading the discussion and following the music scores given as examples. If you have perfect pitch while sight reading music, you might find the musical examples disconcerting. They are all (I think) scored in treble clef, mostly within the staff, even though most of the performers are men with tenor or lower pitched voices. I also had to look up a reference or two on guitar tuning and techniques to follow parts of Charters' analysis.