Dale, Margaret
Crest Publishing House, 1999
Buku Referensi
Armstrong, Michael
Kogan Page, 2001
Buku Teks
Armstrong, Michael
Kogan Page, 2001
Buku Teks
Armstrong, Michael
[Publisher not identified], [Date of publication not identified]
Buku Teks SO
Haines, Helen E.
Columbia University Press, 1950
Buku Teks SO