Dowton, Paul F.
Springer , 2009
Buku Teks
The Conference Board; editor, Edward C. Bursk
The Free Press, 1973
Buku Teks
Ford, Michael J.
George Allen and Unwin, 1982
Buku Teks
Pearce, Fred
Dorling Kindersley, 2002
Buku Teks
edited by Kenneth B. Storey and Karen K. Tanino
CAB International, 2012
Buku Teks