Wharley, W. Gordon
Harper & Row, 1964
Buku Teks
Marslan, Douglas, author
Holt, Rinehart and Winston, 1964
Buku Teks
Clegg, C. J.
John Murray, 1994
Buku Referensi
Mertens, Thomas R., author
Burgess, 1968
Buku Teks
Buffaloe, Neal D., author
Prentice-Hall, 1967
Buku Teks