Perez de Cuellar, Javier, author
United Nation, 1989
Buku Teks
Morton, Rogers C. B.
U.S. Department of Commerce, 1975
Buku Teks
Fortin, Jane, author
Cambridge University, 2005
Buku Teks
Roherty, James M.
University of Miami Press, 1970
Buku Teks
Okolie, Charles Chukwuma, author
Frederick A. Praeger, 1975
Buku Teks