Basler, Roy P.
Evergreen Books Ltd, 1961
Buku Teks
Donald, David Herbert, author
Vintage Books, 2001
Buku Teks
Carnegie, Dale
New York :Dake Carnegie & associates, Inc. ,1999,
Buku Teks
Randall, J.G.
Dodd, Mead & Company, 1946
Buku Teks
Fawcett Publications, 1961
Buku Teks