Bartholomew, Paul C., author
Littlefield, Adams, 1959
Buku Teks
edited by Robert T. Golembiewski, Frank Gibson, Geoffrey Y. Cornog
Rand McNally College, 1972
Buku Teks
Pfiffner, John M., author
Ronald Press, 1960
Buku Teks
Denhardt, Robert B., author
Harcourt College, 1999
Buku Teks
Henry, Nicholas
Prentice-Hall, 1975
Buku Teks