Ross, Janet
Harper & Row , 1965
Buku Teks
Chabner, Davi-Ellen
W.B. saunders , 2001
Buku Teks
Peer, Willie van
Routledge, 1991
Buku Teks
edited by Jonathan Culpeper, Mick Short, and Peter Verdonk
Routledge, 1998
Buku Teks
Ryan, William
Learning System of Company, 1971
Buku Teks