Arnot, Dorothy S.
Macmillan, 1962
Buku Teks
Kraus, Michael, author
University of Oklahoma Press, 1953
Buku Teks
Hofstadler, Richard, author
Prentice-Hall, 1895
Buku Teks
Angle, Paul M.
Rand McNally , 1958
Buku Teks
Schwartz, Bernard, author
McGraw-Hill, 1974
Buku Teks