The best book on Girty (and his brothers) remains Consul Willshire Butterfield, History of the Girtys (1890), although it is sometimes difficult to read. A newer work is Thomas Boyd, Simon Girty: The White Savage (1928). Richard Elwell Banta, The Ohio (1949), has a solid biographical sketch of Girty. For general historical background see Dale Van Every, A Company of Heroes: The American Frontier, 1775-1783 (1962).
Additional Biography Sources
Truman, Timothy, Wilderness: the true story of Simon Girty, the renegade, Lancaster, Pa.: 4 Winds Pub. Group, 1989.
