Chapman, Robert L.
London Pan Books 1988
Buku Referensi
Lutz, William D.
Cambridge University Press, 1994
Buku Referensi
N. A. L. Signet Books,
Buku Referensi
Bloomsbury Publishing Limited, 1993
Buku Referensi
Burton, William.
Simon & Schuster Macmillan, 1992
Buku Teks SO