British Broadcasting Corporation , 1979
Buku Teks
Stapley, Neil F.
Woodhead-Faulkner, in association with Laing & Cruickshank , 1986
Buku Teks
Coase, R.H., author
Longmans Green, 1950
Buku Teks
Norris, Pippa, editor
Cambridge University Press, 1994
eBooks
Williamson, J.A.
Macmillan, 1965
Buku Teks