Bob Cotman makes a valid point about books after 1945 (WW2 era); and some even before then. Some collectors seem to think that the sources were uncontaminated by contacts, insulated in a time capsule from the past. Schools, migration from the south to the north to find work and the backflow of ideas and information, the radio, the growth of prison farms, etc., strongly affected (impacted the modern word) local cultures.

Two very important sources not listed:

Perrow, E. C., 1911-1915, "Songs and Rhymes from the South, Jour. Amer. Folklore, vol. 25, 137-155; vol. 26, 123-173; vol. 28, 128-190. Thanks to www.immortalia.com, these articles are on line and may be downloaded.
Some 275 songs or fragments are in this collection, many from black communities.
Immortalia

Krehbiel, H. E., 1913 (and reprints), "Afro-American Folk-Songs." Especially useful for songs of the Black Creoles (from areas where the Catholic and Anglican religions were strong), which are often closer to African roots than the songs preserved from areas with Protestant slave holders. In spite of the 1913 date, many were collected in the 1880s.
Paperback reprints are reasonable.