Oxford University Press, 1996
Buku Referensi
Jones, David J.
Second Back Row Press, 1981
Buku Teks
Paxton, John, author
J.M. Dent & Sons, 1992
Buku Referensi
Brown, Wendy E., author
McGraw-Hill, 1993
Buku Referensi
Vaccari, 490
Main Bookseller, 1950
Buku Teks