Covey, Sean
A Fireside Book, 2004
Buku Teks
Covey, Sean
[publisher not identified], [date of publication no identified]
Buku Teks
Ashar Sunyoto Munandar
UI-Press, 2006
Buku Teks
Steckle, Lynde C.
Happer , 1957
Buku Teks
Bass, Bernard M., editor
Allyn and Bacon, 1979
Buku Teks