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
Peer, Willie van
Routledge, 1991
Buku Teks SO