edited Brian M. du Toit
Westview Press, 1978
Buku Teks SO
Amoo, Sam G.
United Nations Development Programme, 1997
Buku Teks
edited by Nancie L. Gonzalez and Carolyn S. McCommon
Westview Press , 1989
Buku Teks SO
edited by Karl Cordell
Routledge, 1999
Buku Teks SO
Hance, William A.
Columbia University Press, 1970
Buku Teks SO