Colarik, Andrew M
IDEA Group, 2006
Buku Teks
Norbert Eschborn; Joyce Holmes Richardson; Henriette Litta
Konkrad-Adenauer-Stiftung, 2005
Buku Teks SO
John Pearse.
Wiley Balckwell, 2015
Buku Teks
Gough, Ian
Macmillan, 1992
Buku Teks SO
Carnoy, Martin
Princeton University press, 1984
Buku Teks SO