Waldo, Dwight, author
Ronald Press, 1948
Buku Teks
Appleby, Paul H., author
University of Alabama, 1949
Buku Teks
Charlesworth, James C. (James Clyde), 1900-, author
Harper, 1951
Buku Teks
edited by Robert T. Golembiewski, Frank Gibson, Geoffrey Y. Cornog
Rand McNally College, 1972
Buku Teks
Denhardt, Robert B., author
Harcourt College, 1999
Buku Teks