USAID, 2003
Buku Teks
Twisk, Jos W.R., author
Cambridge University Press, 2006
Buku Teks
Cunningham, James P., author
Kogan Page, 1973
Buku Teks
Barnet, Sylvan, author
Little, Brown, 1980
Buku Teks
Tempest, P.J., author
Pergamon Press, 1981
Buku Teks