Ur, Penny, author
Cambridge University Press, 1991
Buku Teks
Ur, Penny, author
Cambridge University Press, 1999
Buku Referensi
Nunan, David, author
Pearson Education, 2000
Buku Referensi
Christopher Titford edited
Gunter Narr, 1985
Buku Teks
Gebhard, Jerry G., author
Cambridge University Press, 1999
Buku Teks