Tobias, Andrew P.
Simon & Schuster , 1991
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Sydney Hicks,Smith & Sons Pty LTD 1973,
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Wilson, Jerry S., author
American Management Association;, 2008
eBooks
WordPerfect Corporation, 1992
Buku Referensi
Katz, William A., 1924-, author
Holt, Rinehart and Winston, 1979
Buku Teks