Oklahoma Historical Society
Encyclopedia of Oklahoma History and Culture: Arapaho, Southern
A short history of the Arapaho. It explains how they came to be split into their various divisions, the main ones being the Northern and the Southern Arapaho.
Legends of America
Legends of America: Arapaho Great Buffalo Hunters of the Plains
A very good overview of Arapaho history. It explains how the Arapaho split into the Northern and Southern Arapaho tribes and their relationships with the Cheyenne and the Sioux.