Whiting, Percy H.
Dale Carnegie & Associates, Inc., 1979
Buku Teks
Sellars III, David
The Dryden Press, 1992
Buku Teks
Crissy, W.J.E.
John Wiley & Sons, 1977
Buku Teks
Kurtz, David L., author
Business Publications, Inc., 1982
Buku Teks
Russell, Frederick A.
McGraw-Hill Publishing Company, 1988
Buku Teks