Christopher, E.P., author
McGraw-Hill, 1958
Buku Teks
Adams, C.R., author
Butterworth-Heinemann, 1994
Buku Teks
Cambridge University Press, 2014
Buku Teks
Hubbard, L. Ron, 1911-1986, author
Bridge Publications, Inc., 2012
Buku Referensi
Artikel Jurnal