Oxford reference
Oxford University Press, 1990
Buku Referensi
Collin, P. H., author
Peter Collin Publishing, 2001
Buku Teks
Greener, Michael, author
Penguin books, 1994
Buku Referensi
Peter Salim, author
Modern English Press, 1993
Buku Referensi
Pass, Chirtopher, author
Erlangga, 1997
Buku Referensi