Pious, Richard M
Basic Books, 1944
Buku Teks
Francois, William E.
Grid Inc., 1977
Buku Teks
Graber, Doris A., author
Congressional Quarterly Press, 1980
Buku Teks
Jones, Marsha, author
Palgrave, 1999
Buku Teks
editor, Joseph F. Littell
McDougal, Littel and Company, 1976
Buku Teks