Brookings institution, 1949
Buku Teks
Seligman, Eustace, author
New York University Press, 1956.
Buku Teks
Treat, Payson J., author
Peter Smith, 1963
Buku Teks
Brokings Institution, 1950
Buku Teks
Hartmann, Frederick H., author
Macmillan, 1985
Buku Teks