Willborn, Steven L.
The Michie Company, 1993
Buku Teks
Harper, Michael C
Little, Brown and Company, 1996
Buku Teks
McDonald, Stephen L.
The Brookings Institution , 1963
Buku Teks
Tomain, Joseph P.
West Publishing Company, 1981
Buku Teks
Posin, Daniel Q.
St. Paul Minnesota, 1993
Buku Teks