The Johns Hopkins University Press , 1972
Buku Teks
Pritchett, C. Herman
University of North California Press, 1943
Buku Teks
Boxill, Roger
Macmillan, 1994
Buku Teks
Todaro, Michael P.
Longman, 1989
Buku Teks
Oxford University Press, 1972
Buku Referensi