Powell, Ken, author
Macmillan Press, 1977
Buku Teks
Cook, Chris, author
Macmillan, 1975
Buku Referensi
Cook, Chris, author
Macmillan, 1980
Buku Teks
Kaplan, Jeffrey P., author
Prentice Hall International, 1995
Buku Teks
Clark, G. Kitson.
SCM Press , 1950
Buku Teks