Robinson, Marshall A., author
Brookings Institution, 1962
Buku Teks
edited by David Greenaway, Michael Bleaney, and Ian Stewart
Routledge, 1991
Buku Referensi
Macgregor, D.H., author
Oxford University Press, 1949
Buku Teks
Heyne, Paul T., author
Prentice-Hall, 2000
Buku Teks
Heyne, Paul T., author
Science Research Associates, 1973
Buku Teks