Junker, Buford H.
University of Chicago Press, 1960
Buku Teks
Harrison, Brigid C.
Thomson, 2008
Buku Teks
Blackman, Sheldon
John Wiley & Sons, 1971
Buku Teks
Townsend, John C.
McGraw-Hill , 1953
Buku Teks
Duverger, Maurice
California University of california 1967,
Buku Teks