Gilster, Paul, author
John Wiley & Sons, 1995
Buku Teks
British Broadcasting Corporation , 1979
Buku Teks
UK : The Association of Commonwealth Universities, 1988,
Buku Referensi
Alexander, Harriet Semmes, 1949-, author
Swallow Press, 1986
Buku Referensi
Dobson, Frank S., author
Richmond, 1992
Buku Teks