Hewings, Martin
Cambridge University Press, 2005
Buku Teks SO
Artikel Jurnal
McCarthy, Michael
Erlangga, 2002
Buku Teks SO
Azar, Betty Schrampfer, 1941-
Prentice Hall Regents, 1995
Buku Teks SO
Azar, Betty Schrampfer, 1941-
Prentice-Hall, 1985
Buku Teks SO