Sue Wheeler, David King
Sage Publications, 2001
Buku Teks
Bernard, Janine
Allyn and Bacon , 1998
Buku Teks
Hawkins, Peter, author
Open University Press, 2005
Buku Teks
Her Majesty's Stationery Office, 1955
Buku Teks
Bromhead, Peter
Longman, 1992
Buku Teks