Petersen, William, author
Macmillan, 1969
Buku Teks
Brooks, George E.
Westview Press, 1993
Buku Teks
edited by Betty Jo Irvine
Libraries Unlimited, 1991
Buku Teks
Bowen, Elizabeth, 1899-1973, author
Continental Book Company, 1946
Buku Teks
United Nations, 1999
Buku Teks