Halvorson, Heidi Grant-, 1973-, author
Plume Penguin Group, 2014
Buku Teks
Roberts, Ken
Llewellyn Publications, 1995.
Buku Teks
Adrienne, Carol, author
Hikmah, 2007
Buku Teks
Canfield, Jack, 1944-, author
Gramedia Pustaka Utama, 2006
Buku Teks
Tracy, Brian, author
American Management Association, 2005
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