edited by Nancie L. Gonzalez and Carolyn S. McCommon
Westview Press , 1989
Buku Teks SO
edited Brian M. du Toit
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Buku Teks
Amoo, Sam G.
United Nations Development Programme, 1997
Buku Teks
Hance, William A.
Columbia University Press, 1970
Buku Teks
Browne, G.S.
F.W. Chesire, 1960
Buku Teks