Harmon, Sharon Koomen, author
John Wiley & Sons, 2011
Buku Teks
Traister, John E., author
Reston Publishing Company, 1979
Buku Referensi
Sykes, Peter
Longmans, 1966
Buku Referensi
Foundation for Rural Education and Development (FRED) (June 12, 1999, 1999
Buku Teks
Hughes, Fredrick W., author
Prentice-Hall, 1983
Buku Referensi