Clegg, C.J.
John Murray, 1994
Buku Referensi
Marslan, Douglas
Holt, Rinehart and Winston, 1964
Buku Teks SO
Mertens, Thomas R.
Burgess, 1968
Buku Teks SO
Buffaloe, Neal D.
Prentice-Hall, 1967
Buku Teks
by Robert J. Brooker ... [and others]
McGraw-Hill, 2008
Buku Teks