Povey, John F.
Holt, Rinehart and Winston, 1984
Buku Teks
Hornby, Albert Sydney, author
Oxford University Press, 1991
Buku Referensi
French, F. G., author
Oxford University Press, 1947
Buku Teks
Drummond, Gordon, author
Essex Longman, 1972
Buku Teks
Bradford, Barbara
Cambridge University Press, 1988
Buku Teks