Cammack, Paul Anthony
Leicester University Press, 1997
Buku Teks SO
Hettne, Bjorn 1939-
Longman Scientific & Technical , 1995
Buku Teks
Critchfield, Howard J.
Prentice-Hall, 1979
Buku Teks
Deegan, Heather
Routledge, 1996
Buku Teks SO
Bryant, Raymond L.
Routledge, 1997
Buku Teks