Downes, John
Barron's Educational Series, 1998
Buku Referensi
Hildreth, Sandra S.
Dearborn Financial Publishing, 1988
Buku Referensi
Oxford University Press, 1993
Buku Referensi
Goode, Walter
Cambridge University Press, 2007
Buku Referensi
Cuddon, J.A.
Penguin Books, 1979
Buku Referensi