Here are some books that are considered classics in the study of 19th and early 20th century African American* folk music.

Slave Songs of the United States: The Classic 1867 Anthology [William Francis Allen, Charles Pickard Ware, Lucy McKim Garrison]
http://books.google.com/books/about/Slave_Songs_of_the_United_States.html?id=yrsTaGBpk6UC

http://www.traditionalmusic.co.uk/folk-song-an/
"Folk Song Of The American Negro: An investigation into traditional American negro folk songs"... By John Wesley Work, A.M. Professor Of Latin And History Fisk University c1915 [online book]

Negro Folk Rhymes Wise and Otherwise by Thomas W. Talley
[originally published by Macmillan Press in 1922
http://www.gutenberg.org/files/27195/27195-h/27195-h.htm
[EBook]

On The Trail Of Negro Folk-Songs By Dorothy Scarborough Assisted By Ola Lee Quiledge Copyright, 1925 By Harvard University Press [online book]