West, Michael D.
Longmans, Green and Co., 1951
Buku Teks
Heinemann Educational Books, 1981
Buku Teks
French, F. G.
Oxford University Press, 1947
Buku Teks
Lambirth, Andrew
Learning Matters, 2005
Buku Teks
Encyclopaedia Britannica, 1981
Buku Referensi