Bellinger, William Kenneth
Routledge, 2007
Buku Teks SO
Luksetich, William A.
Little, Brown, 1982
Buku Teks
World Bank
Oxford University Press for the World Bank, 1993
Buku Teks
Bromley, Daniel W., 1940-
Basil Blackwell, 1989
Buku Teks SO
Oxford University Press , 1993
Buku Teks SO