Hyland, Ken, author
Longman, 2002
Buku Teks
Buys, William E.
National Textbook Cmopany, 1994
Buku Referensi
edited by James Flood, Shirley Brice Heath, and Diane Lapp
MacMillan Library Reference USA, 1996
Buku Referensi
Katzen, May
Unesco Press, 1975
Buku Teks
Artikel Jurnal