Insor, D., author
Frederick A.Praeger, 1963
Buku Teks
South, Stanley
Academic Press, 1977
Buku Teks
New York Academic Press 1977
Buku Teks
Gruhn, Ruth, author
Pacatello, 1961
Buku Teks
editor, Gunter Schramm and Jeremy J. Warford
The World Bank, 1991
Buku Teks