Blumenson, Martin
Angkasa, 1975
Buku Teks
Pushies, Fred, author
American Management Association, 2009
eBooks
Haythornthwaite, Philip J.
Poole, Dorset Fiirebird Books 1991,
Buku Teks
Masters, John
The Reprint Society, 1956
Buku Teks
Morris, David Joseph, author
Pergamon Press, 1983
Buku Teks