Hubbard, L. Ron, 1911-1986, author
Bridge Publications, 2012
Buku Referensi
Carrel, Alexis, author
Penguin Books, 1967
Buku Teks
De Blij, Harm J., author
John Wiley & Sons, 1994
Buku Teks
De Blij, Harm J., author
John Wiley & Sons, 1997
Buku Teks
Carnegie, Dale, 1888-1955, author
New York :Dake Carnegie & associates, Inc. ,1999,
Buku Teks