As for the prewar blues and gospel records, Robert M.W. Dixon, John Godrich & Howard W. Rye's Blues and Gospel Records 1890-1943, 4th edition (Oxford, 1997)is a great source of information. It contains unissued ones (including LOC recordings), LPs, CDs, and some videos, too. There're interesting entries: Unidentified singers and groups, Unknown artists, Street cries of Charleston, Negro songs of protest, and so on. Michael Ruppli's The Chess Labels, 2 vols. (Greenwood, 1983) should be added. Another reference book is Paul Vernon, African-American Blues, Rhythm and Blues, Gospel and Zydeco on Film and Video, 1926-1997 (Ashgate, 1999). Not so comprehensive, but a good guide.

~Masato