Roucek, Joseph S., author
Thomas Y. Crowell Company, 1950
Buku Teks
Strong, C. F.
Sidgwick and Jackson, 1972
Buku Teks
Field, G. Lowell, author
Cornell University Press, 1967
Buku Teks
Easton, David, author
Prentice-Hall, 1965
Buku Teks
Roelofs, H. Mark
The Dorsey press, 1968
Buku Teks