Hill, L.A.
Longmans, Green and Co., 1959
Buku Teks
Olding, Simon
Her Majesty's Stationary Office, 1989
Buku Referensi
Phillips, Estelle M.
Open University Press, 1994
Buku Teks
Phillips, Estelle M.
Open University Press, 2005
Buku Teks
Muinzer, Genevieve
Routledge and Kegan Paul, 1987
Buku Teks