Schagrin, Morton L.
Random House, 1968
Buku Teks SO
Wales, Katie
Longman , 1991
Buku Referensi
edited by Jonathan Culpeper, Mick Short, and Peter Verdonk
Routledge, 1998
Buku Teks SO
Ross, Janet
Harper & Row , 1965
Buku Teks SO
Chabner, Davi-Ellen
W.B. saunders , 2001
Buku Teks SO