The Open University
Routledge, 2006
Buku Teks
Smoke, Trudy.
Cambridge University Press,, 2006
Buku Teks
Brooks, Brian S.
Bedford/St. Martins,, 2002
Buku Teks
Joyce, Helen de Silva
Phoenix Education, 2000
Buku Teks
Mills, Paul
Routledge, 2006
Buku Teks