Hudson, Robert V., author
Garland publishing, 1987
Buku Referensi
editor, Joseph F. Littell
McDougal, Littel and Company, 1976
Buku Teks
Vivian, John, author
Pearson and AB, 2007
Buku Teks
edited by Wilbur Schramm
University of Illnois Press, 1954
Buku Teks
Dominick, Joseph R., author
McGraw-Hill, 1993
Buku Teks