Gunther, John, 1901-1970
Hamish Hamilton, 1939
Buku Referensi
Eleqant, Robert
Avon Books, 1991
Buku Teks SO
Sidhu, Gursharan S.
Addison-Wesley, 1990
Buku Teks SO
Newby, Julian, 1957-
Routledge, 1997
Buku Teks
Downs, Anthony
Little, Brown and Co., 1967
Buku Teks