Sylvia, Ronald D.
Harcourt College Publisher, 2002
Buku Teks
Cayer, N. Joseph
Thomson wadsworth, 2004
Buku Teks
Simon, Herbert A.
Alfred A. Knopf, 1973
Buku Teks
Bartholomew, Paul C.
Littlefield, Adams, 1959
Buku Teks
edited by Robert T. Golembiewski, Frank Gibson, Geoffrey Y. Cornog
Rand McNally College, 1972
Buku Teks