Lala, Russi M.
Vision Books, 1987
Buku Teks
Harvard Business School
Harvard Business Review Press, 2011
Buku Teks
Lawson, Karen
ASTD Press, 2008
Buku Teks
Burby, Raymond J., author
Liberty, [Date of publication not identified]
Buku Teks
Iowa University 1971 ,
Buku Teks