Unrau, Harlan D.
National Park Service, 1976
Buku Teks
edited by Edmund S. Morgan
Prentice-Hall, 1965
Buku Teks
Morris, Richard B., author
Van Nostrand Reinhold, 1955
Buku Teks
Miller, John C.
Stanford University Press, 1965
Buku Teks
edited by Jack P. Greene, J.R. Pole
Blackwell , 1994
Buku Referensi