Walton, Francis
New York Macmillan 1956,
Buku Teks
United Nations, 1983
Buku Teks
Clark, Kim B., author
Harvard Business Scholl Press, 1991
Buku Teks
Grosvenor Press International, T.t.
Buku Teks
Marx, Daniel, author
Priceton University Press, 1953
Buku Teks