Dyke, Vernon van
Nelson, 1992
Buku Teks SO
Merriam, Charles E. (Charles Edward)
University of Chicago Press, 1957
Buku Teks
Tansey, Stephen D.
Routledge, 1996
Buku Teks SO
Combs, James E.
Macmillan, 1984
Buku Teks SO
Pye, Keith
Stanley Thornes , 1992
Buku Teks SO