Bailey, Stephen K., author
Henry Holt, 1957
Buku Teks
Moore, John A.
Macmillan Publishing Company, 1985
Buku Teks
Chafe, William Henry
Oxford University Press, 1995
Buku Teks
Kendall/Hunt, 1982
Buku Teks
Millspaugh, Arthur Chester, 1883-1955
The Brookings Institution, 1942
Buku Teks