Bowser, eileen, author
Macmillan Library reference USA, 1990
Buku Referensi
Ovidiu Nicolescu, Lester lloyd-Reason
World Scientific Publishing Co., 2015
Buku Teks
Certoma, G. Leroy
Butterworth, 1985
Buku Teks
Borrie, W. D., author
F.W. Chesire, 1954
Buku Teks
London English University Press 1958
Buku Teks