The America University, [date of publication not identified]
Buku Teks
Holloway, William Vernon
McGraw-Hill , 1951
Buku Teks
Younger, Richard D.
Brown University Press, 1963
Buku Teks
Shotwell, James T.
New York, N.Y. MacMillan Co. 1940,
Buku Teks
Burns, James McGregor
Prentice-Hall, 1972
Buku Teks SO