Allen, R.G.D.
Macmillan, 1980
Buku Teks
Vandermeulen, Alice John, 1918-
Prentice-Hall, 1956
Buku Teks
by United Nation
United Nations, 1991
Buku Teks
edited by Ole Bruun and Arne Kalland
Surrey Curzon Press , 1996
Buku Teks
Her Majesty's Stationery Office, 1987
Buku Referensi