Walker, E. Ronald
University of Chicago Press , 1947
Buku Teks
Boulding, Kenneth E. (Kenneth Ewart), 1910-1993, author
Prentice-Hall, 1958
Buku Teks
Cross, Rod
Martin Robertson , 1982
Buku Teks
Fabella, Armand V.
University of the Philippines, 1968
Buku Teks
Lewis, W. Arthur
Harper & Row, 1966
Buku Teks