Barratt, William B.
Aldine House, 1994
Buku Teks
Coultas, Barbara
Teach Yourself Books, 1990
Buku Teks SO
Tippett-Spirtou, Sandy
Routledge, 1999
Buku Teks SO
Graham, Gaelle
Hodder & Stoughton, 1991
Buku Teks SO
consultant editor, D.E. Hathway
Harrap, 1985
Buku Referensi