Toynbee, Arnold J., author
Oxford University Press , 1948
Buku Referensi
edited by George A. Vanasse
Academic Press, 1981
Buku Teks
Owen, George E., author
Harper and Row, 1967
Buku Teks
International Society for Educational Information
ISEI, 1987
Buku Teks
Virtue and Company Limited , 1977
Buku Teks