Cambridge University Press, 2016
eBooks
Virtue, Doreen, author
Hay House, Inc, 2001
Buku Teks
Simmons, Christopher, 1973-, author
Rockport, 2006
Buku Referensi
Mahnke, Frank H.
John Wiley & Sons Inc., 1987
Buku Teks
Copestick, Joanna, author
Page One , 1998
Buku Referensi