Vercruijsse, Emile
Institute of Social Studies , 1988
Buku Teks
Igbozurike, Martin
Scandinavian Institute of African Studies, 1976
Buku Teks
Balinger Pub. Co., 1974
Buku Teks
Potten, A.J. van, author
Rabobank International Group, 1966
Buku Teks
Potten, A.J. van, author
Rabobank International Group, 1996
Buku Teks