Bien, Thomas
Wisdom Publications, 2006
Buku Teks SO
Cohen, Monique
Sage, 2000
Buku Referensi
Nelson-Jones, Richard
Thousand Oaks, California: Sage, 2014
Buku Teks SO
Yerbury, Hilary
University of Technology, 1992
Buku Referensi
Brown, Adrian
Facet Publishing, 2006
Buku Teks SO