Hoffding, Harold, author
Macmillan, 1955
Buku Teks
Ayres, C.E.
Universty of Taxas Press , 1961
Buku Teks
Muhs, Karl
Duncker & Humblot, 1950
Buku Teks
Ople, Blas F., author
The Institute for Public Policy, 2001
Buku Teks
Willener, Alfred
Pantheon Books, 1970
Buku Teks