Rawlins, Karen
MacMillan Press , 1993
Buku Teks
Higgins, James M.
Random House Business Division, 1982
Buku Teks
Boyd, Claire
Wiley Blackwell, 2014
Buku Teks
editor, Owen Hargie
New York , 2006
Buku Referensi
Johnson, Stuart
Oxford Univesity Press, 2009
Buku Teks