McLeod, John, author
McGraw-hill Open University press, 2012
Buku Teks
Bien, Thomas, author
Wisdom Publications, 2006
Buku Teks
Cohen, Monique, author
Sage, 2000
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Nelson-Jones, Richard, author
Thousand Oaks, California: SAGE, 2014
Buku Teks
Yerbury, Hilary, author
University of Technology, 1992
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