Covey, Stephen R., author
Simon & Schuster , 1990
Buku Teks
Monath, Norman
Pink Books, 2004
Buku Teks
Tracy, Brian, author
John Wiley & Sons, 2003
Buku Teks
McGrath, John, author
Gramedia Pustaka Utama, 2006
Buku Teks
Chant, Bent, author
Avon Books, 1994
Buku Teks