Greener, Michael
Penguin books, 1994
Buku Referensi
Pass, Chirtopher
Erlangga, 1997
Buku Referensi
Collin, P. H.
Peter Collin Publishing, 2001
Buku Teks
De Vries, Mary Ann
Prentice-Hall, 1995
Buku Referensi
Oxford reference
Oxford University Press, 1990
Buku Referensi