Mosher, Frederick C.
Oxford University Press, 1968
Buku Teks
Wheare, K. C.
Oxford University Press, 1963
Buku Teks
Banfield, Edward C., author
Free Press, 1951
Buku Teks
Noyes, Arthur P.
W.B. Saunders , 1958
Buku Teks
Roberts, Arthur D.
McGraw-Hill , 1968
Buku Teks