Benns, Lee F., author
Appleton-Century-Crofts, 1955
Buku Teks
compiled by Jonathan French Scott and Alexander Baltzly
F.S. Crofts & Co., 1930
Buku Teks
Stearns, Peter N., author
Macmillan, 1967
Buku Teks
Smith, Bonnie G., author
D.C. Heath, 1989
Buku Teks
Burns, Arthur F., author
National Bureau of Economics Reseach, 1934
Buku Teks