Elliot, Catherine, author
Longman , 2001
Buku Teks
McKendrick, Ewan, author
Macmillan, 1994
Buku Teks
Kapteyn, P.J.G., author
Sweet and Maxwell, 1973
Buku Teks
Mulgan, Richard, author
Oxford University Press, 1989
Buku Teks
University of California Press, 1947
Buku Teks