Steckle, Lynde C.
Happer , 1957
Buku Teks
Covey, Stephen R., author
A Fireside Book, 1989
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Strongman, Kenneth T., author
John Wiley & Sons, 2006
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Biehler, Robert F.
Houghton Miffiln, [date of publication not identified]
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Lieberman, David J., author
Serambi Ilmu Semesta, 2011
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