Schneider, Gerhard W., author
Ronald , 1974
Buku Teks
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