Whiting, Percy H.
Dale Carnegie & Associates, Inc., 1979
Buku Teks
Nixon, H.K.
McGraw-Hill, 1931
Buku Teks
Allen, Peter, author
Pitman, 1998
Buku Teks
Russell, Frederick A.
McGraw-Hill Publishing Company, 1988
Buku Teks
Crissy, W.J.E.
John Wiley & Sons, 1977
Buku Teks