Stuart, Graham Henry, 1887-, author
Appleton-Century-Crofts, 1955
Buku Teks
Westview Press, 1980
Buku Teks
Risjord, Norman K., author
Prentice-Hall, 1985
Buku Teks
Rand McNally and Company.
Rand Mcnally & Company, 2006
Buku Teks
Washington U.S. Department of Health, Education, and Welfare 1955
Buku Teks