Denhardt, Robert B.
Harcourt College, 1999
Buku Teks
Denhardt, Robert B.
Wadsworth and Cengage Learning, 2014
Buku Teks
Herring, E. Pendleton
McGraw-Hill, 1936
Buku Teks
Pfiffner, John M.
Ronald Press, 1960
Buku Teks
edited by Robert T. Golembiewski, Frank Gibson, Geoffrey Y. Cornog
Rand McNally College, 1972
Buku Teks