West, Michael D.
Longmans, Green and Co., 1951
Buku Teks
Heinemann Educational Books, 1981
Buku Teks
Lambirth, Andrew
Learning Matters, 2005
Buku Teks SO
Tom Dalzell, Terry Victor, editor
Routledge, 2006
Buku Referensi
Tomasowa, Francien Herlen
Universitas Brawijaya , 2005
Buku Teks SO