Alan Lomax is a terrific place to start, also I would suggest that you research the African roots that are the source all African American music. The oral tradition is illustrated beautifully in a film called The Language You Cry In, (the trailer URL is attached below).
A book called The Garland Handbook of African Music edited by Ruth M. Stone should prove useful in understanding oral tradition and the relationship between African instruments and African American music played on European instruments. This book also gives information about the importance of movement and dance with African music.
The Gullah people of the Southern coast of America offer unique peeks at cultural ties between the continents. If you access the added info below you will find some information about the Gullah and a very good film called Daughters of the Dust. Although this film may not give specific information about music history, it does show things about this unusual group of people that managed to maintain their cultural history because of the isolation of their island homes.
The best sources for information about the contribution of the African-American community to folk music, including the development and revival of folk music in the United States, are the works of the ethno-musicologist Alan Lomax. Not only will you find the written versions of many early folk songs and spirituals but also the actual recordings of hundreds of songs from all parts of the American South and the Caribbean.
Beginning in the 1930s, Lomax began traveling all over the United States--concentrating on the South--in order to record the roots of American folk music, which, in large part, began in the African-American community as spirituals. In many cases, Lomax's recordings constitute the only evidence we have of how some folk songs and spirituals were performed. In addition to recording songs that would have been lost when one or two performers died, Lomax recorded as many variations of songs as he could uncover, giving us the only record of how folk songs reflected the experiences of people from one locale to another.
Based on Lomax's work, you can listen to and write about folk songs and spirituals as they existed not just in the American South but also by state and, sometimes, by areas within a particular state. For example, Lomax recorded as many songs as he could find in various dialects within a state--South Carolina, for example, where he found folk songs and spirituals in what is now referred to as a "low-country dialect."
Many of his most interesting insights into the development of folk music are in Lomax's letters, so it is worth the effort to take a look at an edition of his letters.
Lomax got interested in folk music partly because his father was a well-known folklore specialist, but Lomax's interest in folk music, and African-American music in particular, probably stems from his belief that African-American culture in general had to be preserved before it was lost. And because folk music, especially in the African-American community across the South, was oral rather than written, Lomax understood the urgency of recording as much of this music as possible.
Several books you might want to look at include American Ballads and Folk Songs, The Southern Journey of Alan Lomax, and Alan Lomax: Selected Writings, 1934-1997. In addition, the Library of Congress has a large selection of his actual recordings of folk music, African-American spirituals, and Blues.