Ben, Great Grandfather Abner was listed in the 'Roster of Guerrillas' in "Quantrill and His Civil War Guerrillas" by Carl W. Breihan, Promontory Press, N.Y. (1959). Also - 'Known members of Quantrills, Andersons, and Todd's Guerrillas' - an appendix in "Gray Ghosts of the Confederacy Guerilla Warfare in the West 1861-65" by Richard S. Brownlee, Louisana State University Press 1958.
I also have 3 different published stories that include him as part of Quantrill's band. I'll post them on my tree - tomorrow if I get a chance.