Rawlings, Philip, author
Longman, 1999
Buku Teks
Powell, Edward, author
Oxford University Press, 1989
Buku Teks
Mike Fitzgerald, Gregor McLennan and Jennie Pawson at the Open University.
Routledge in association with Open University Press, 2001
Buku Teks
Rawlings, Philip, author
Longman, 1999
Buku Teks
Roth, Mitchel P., author
Cengage Learning , 2010
Buku Teks