Seal, Bernard
Cambridge , 1999
Buku Referensi
Priestley, Philip
Tavistock Publications, 1983
Buku Teks
Haarman, Louann
Oxford University Press, 1989
Buku Teks
Langan, John, 1942-
McGraw-Hill, 1992
Buku Teks
Spears, Deanne Milan
McGraw-Hill, 2000
Buku Teks