Covey, Stephen R., author
A Fireside Book, 1989
Buku Teks
Covey, Sean, author
[publisher not identified], [date of publication no identified]
Buku Teks
Covey, Sean, author
A Fireside Book, 2004
Buku Teks
Ashar Sunyoto Munandar, author
UI-Press, 2006
Buku Teks
Nicolson, Paula, author
MacMillan Education, 1986
Buku Teks