Cohen, Jack
Penguins Books, 1994
Buku Teks SO
Cotterill, Rodney
Cambridge University Press, 1989
Buku Referensi
edited by Gerald W. Whitted
Pan American World Airways, 1971
Buku Referensi
O`Donnell, Sandra Martin
Prentice-Hall, 1994
Buku Teks SO
edited by Ian Harris ... [and others]
Longman Current Affairs, 1992
Buku Teks SO