Croft, Kenneth
Mass. Winthrop , 1972
Buku Teks
Protherough, Robert
Open University Press, 1991
Buku Teks
McArthur, Tom (Thomas Burns), author
Cambridge University Press, 1983
Buku Teks
Sambrook, G.A., author
Longmans, Green, 1947
Buku Teks
Craswell, Gail, author
SAGE, 2005
Buku Teks