Webster, John
Macmillan, 1975
Buku Teks SO
Weir, Alison
Ballantine Books , 2001
Buku Teks SO
Danbury : Lexicon Publications, Inc., 2004,
Buku Referensi
Danbury : Lexicon Publications, Inc., 2004,
Buku Referensi
Scott-Kilvert, Ian
Longmans, Green, 1970
Buku Teks SO