Singh, Robert
Polity Press, 2002
Buku Teks
[publisher not identified], 1968
Buku Teks
Simpson, David, author
Oxford University Press , 1986
Buku Teks
Stephen J. Wayne [et all.]
Thomson, 2005
Buku Teks
Barone, Michael, author
National Journal, 1992
Buku Referensi