Progressio, 1972
Buku Teks
Scott, James C., author
Yale University Press, 1976
Buku Teks
Fifield, Russell H., author
Praeger , 1963
Buku Teks
editor, Syed Farid Alatas
National Library, 2010
Buku Referensi
Grubel, Herbert G., author
Lexington Books, 1973
Buku Teks