Richardson, Joanna
The British Council by Longmans, Green, 1960
Buku Teks
Cambridge University Press, 2010
eBooks
Little, Brown and Company , 1998
Buku Teks
Kelly, Lois, 1955- , author
American Management Association, 2007
eBooks
Irving, John
Ballantine Books, 1989
Buku Teks