Hajnal, Peter I., author
New York : Oceana Publications, 1978,
Buku Referensi
by United Nations
United Nations, [Date of publication not identified]
Buku Teks
Goodrich, Leland M., author
Thomas Y. Crowell, 1961
Buku Teks
Bailey, Sydney D., author
Marnes & Noble Books, 1995
Buku Teks
by United Nations
Departement of Public Information - United Nations, 1956
Buku Teks