Held, Colbert C.
Westview Press, 2014
Buku Teks SO
Palmer, Monte
Thomson Wadsworth, 2007
Buku Teks SO
Harari, Maurice
Prentice-Hall, 1962
Buku Teks
Bill, James A.
Littel Brown and Company, 1984
Buku Teks
Bill, James A.
Little, Brown, 1979
Buku Teks SO