Whiting, Percy H.
Dale Carnegie & Associates, Inc., 1979
Buku Teks
Manning, Gerald L., author
Prentice Hall International, 1998
Buku Teks
Kahn, George N.
Gramedia Pustaka Utama, 1996
Buku Teks
Sellars III, David
The Dryden Press, 1992
Buku Teks
Williams, Alan
McGraw-Hill, 1983
Buku Teks