Shanks, Michael, author
Routledge , 1992
Buku Teks
Stiebing, William H.
Oxford University Press, 1993
Buku Teks
Gill, Richard T. (Richard Thomas), 1927-2010, author
Prentice-Hall, 1963, 1967
Buku Teks
Gladstone, W. E.
London John Murray 1879
Buku Teks
Joyce, Rosaleen, author
McGraw-Hill, 2012
Buku Teks