Levison, Arnold B.
Pegasus, 1974
Buku Teks
Boyd, Maurice
Allyn and Bacon, Inc., 1969
Buku Teks
Wisdom, J. O.
Avebury, 1987
Buku Teks
Erickson, Bonnie H., author
Open University Press, 1977
Buku Teks
Duverger, Maurice
California University of california 1967,
Buku Teks