Heaton, J.B.
Longman Croup UK Limited, 1988
Buku Teks
Stone, Linton
Macmillan, 1970
Buku Teks
Blundell, Jon, author
Oxford University Press, 1982
Buku Teks
Gerber, Philip L., author
Random House, 1959
Buku Teks
Burton, Samuel Holroyd, author
Macmillan, 1995
Buku Teks