Murray, William G.
The Iowa State University, 1963
Buku Teks
Helmberger, Peter G., author
McGraw-Hill, 1991
Buku Teks
edited by Olaf F. Larson, Edward O. Moe, and Julie N. Zimmerman
Westview Press, 1992
Buku Teks
edited by Don E. Hadwiger and Ross B Talbot
Academy of Political Science, 1982
Buku Teks
Shepherd, Geoffrey S., author
Iowa State University Press, 1968
Buku Teks