Richard Mabey, editor
Penguin , 1991
Buku Referensi
Keogh, James Edward
McGraw-Hill, 1977
Buku Teks
edited by Cristina Paredes Beniftez
Page one, 2011
Buku Referensi
Dollin, Stuart, author
The Templar Company, 1992
Buku Referensi
Segal, Lloyd M., 1948- , author
American Management Association, 2007
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