Baer, Werner
Richard D. Irwin, 1965
Buku Teks
Lewis, William Arthur, 1915-
University of Ghana by the Ghana Pub, 1971
Buku Teks
Echavarria, Jose Medina
New York, N.Y., Unesco
Buku Teks
edited by E.K. Fisk
Australia National University, 1966
Buku Teks
Teichert, Pedro C.M.
Bureau of Business Research Mississippi University , 1959
Buku Teks