Culver, John H.
Macmillan Publishing Company,
Buku Teks
Brewster, Lawrence G., author
Thomson Wadsworth, 2004
Buku Teks
Brzezinski, Zbigniev K., author
Frederick A. Praeger, 1962
Buku Teks
Dolbeare, Kenneth M., author
D.C. Heath, 1985
Buku Teks
Bell, Charles G.
The Dorsey Press, 1980
Buku Teks