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