Cannadine, David
Papermac, 1996
Buku Teks
Willcox, William B., author
D.C. Heath and Company, 1988
Buku Referensi
Willcocx, William B.
D.C. Health and Company, 1988
Buku Teks
Simpson, W.O., author
Stanley Thornes, 1986
Buku Teks
Macmillan, 1994
Buku Teks