A good start would be these:
The Negro Baseball Leagues: A Photographic History by Phil Dixon
Pictorial Negro League Legends by Robert Retort
The Negro Leagues Book by SABR
When The Game Was Black and White by Chadwick
I compile reference from other smaller books about players or teams which often have good, little known photos in them. Sometimes you can get decent reference from the Afro-American online archives, but the photo quality can be real bad. Basicly I just keep my eyes open for any book on the negro leagues and copy photos I think I might need for my projects. Hope this helps and if you need something specific I'll try to be of help.
My Baseball Card Project:
www.infinitecardset.blogspot.com