Machlup, Fritz, author
The Johns Hopkins Press, 1952
Buku Teks
Hirst, Francis W.
E.P. Dutton & Co , 1906
Buku Teks
Stocking, George W., author
The Twentieth Century Fund , 1949
Buku Teks
Watt, Gary, author
Blackstone Press Limited, 1998
Buku Referensi
Nossaman, Walter L.
Matthew Bender and Company, 1961
Buku Teks