Snow, Donald M., author
Pearson Longman, 2007
Buku Teks
edited by Ali Banuazizi, Myron Weiner
Indiana University Press, 1994
Buku Teks
Dixey, Graham
Newnes, 1994
Buku Teks
Graham, Ian S.
John Wiley & Sons, 1995
Buku Referensi
Graham, Sheilah
The Viking Press, 1967
Buku Teks