Coughlan, Robert
Time Incorporated, 1962
Buku Teks
edited by John C. Caldwell and Chukuka Okonjo
Longmans, Green, 1968
Buku Teks
Marcus, Edward
Bookman Associates, 1960
Buku Teks
Due, John F.
MIT Press, 1963
Buku Teks
Kimble, George H.T.
The Twentieth Century Fund, 1960
Buku Teks