Marks, Trevor E., author
William Ernest, 1992
Buku Teks
Thompson, F.G.
Longmans, Green and Co, Ltd., 1968
Buku Teks
Pethebridge, Keith, author
McGraw-Hill, 2012
Buku Teks
Thompson, F.G.
Longman Scientific and Technical, 1992
Buku Teks
McPartland, Joseph F., author
McGraw-Hill, 1984
Buku Referensi