Cambridge University Press, 2003
eBooks
Collins, James, author
Cambridge University Press , 2005
Buku Teks
Barton, David
Routledge , 1998
Buku Teks
Open University Press, 1997
Buku Teks
Hubbard, L. Ron, 1911-1986, author
Bridge Publications, Inc., 2012
Buku Referensi