Knol, Kenneth M.
Sheil Oil Company, 1977
Buku Teks
edited by R.R. Betts
Royal Institute of International Affairs, 1950
Buku Teks
Tinbergen, Jan, 1903-, author
Yale University Press, 1964
Buku Teks
Rice, Tamara Talbot
Praeger, 1965
Buku Teks
Rodriguez, Mario
Prentice-Hall, 1965
Buku Teks