Wirtz, W. Willard, author
Harper & Row, 1964
Buku Teks
Herring, E. Pendleton, author
McGraw-Hill, 1936
Buku Teks
Schultze, Charles L., author
The Brookings Institution, 1977
Buku Teks
Philip G. Schrag
Little,Brown and Company, 1974
Buku Teks
Gaedeke, Ralph M., author
Canfield Press , 1972
Buku Teks