Aboyade, Ojetunji, author
Praeger, 1966
Buku Teks
Reagan, Michael D., author
Oxford University Press, 1963
Buku Teks
Heilbroner, Robert L., author
Simon & Schuster, 1994
Buku Teks
edited by Harold F. Williamson
Prentice-Hall, 1951
Buku Teks
Robertson, Ross M., author
Harcourt, Brace & World, 1964
Buku Teks