Schmidt, Steven
Prentice-Hall, 1983
Buku Teks
School Government Publishing Co., [date of publication not identified]
Buku Referensi
edited by Janet Moyles
McGraw-Hill, 2007
Buku Teks
British Council, 1991
Buku Referensi
Galloway, David
Longman, 1987
Buku Teks