Some info from other sites.

(1) Blues In the Mississippi Night: An Interview with Memphis Slim, Big Bill Broonzy and Sonny Boy Williamson (Rykodisc, 1990)
Folklorist Alan Lomax recorded this amazing interview in the 1940s with three great bluesmen of this century. It gives a glimpse into how the blues evolved from songs created in response to conditions at prison farms and contract labor camps; songs created to make the back-breaking work in the sun somewhat more tolerable. In this selection, the three men discuss working conditions in a contract labor camp; conditions which, as you'll hear, were not much better than slavery the century before.

From this.

(2) Review of Deep Blues: A Musical and Cultural History of the Mississippi Delta. This was first published by Viking/Penguin in 1981. Robert Palmer edited a video on the field trip with the same title, the content not being the same. Also interesting.

~Masato