Schweizer, Heidi
Ally & Bacon , 1999
Buku Teks
Nunan, David , author
Cambrigde University Press , 1993
Buku Teks
Moore, Kenneth D, author
McGraw--Hill, 2007
Buku Teks
Williams, Robin, author
Peachpit Press, 2000
Buku Teks
Davis, Ray
Lycos Press,, 1997
Buku Referensi