Singh, Robert
Polity Press, 2002
Buku Teks
Frank, Jerome
Princeton University press, 1973
Buku Teks
Jenkins, Jeffrey A.
Jones and Bartlett Publishers LLC , 2011
Buku Teks
Forer, Lois G.
Norton, 1984
Buku Teks
Ginsberg, Morris
Penguin Books, 1965
Buku Teks SO