Perrin, Porter G.
Glenview, 1968
Buku Teks SO
Oxford university press , 1992
Buku Teks
edited by Della Thompson
Oxford University Press, 1996
Buku Referensi
Newby, Michael
Cambridge University Press, 1988
Buku Teks SO
Hornby, Albert Sydney
Oxford University Press, 1984
Buku Referensi