Homan, Paul T.
Harcourt, Brace and Company, 1958
Buku Teks
Boobs-Merrill, 1963
Buku Teks
Salvadori, Massimo, author
Bobbs-Merrill, 1963
Buku Teks
Silberling, Norman J., author
McGraw-Hill, 1943
Buku Teks
Moulton, Harold G., author
Brookings Institution, 1935
Buku Teks