Howlett, Michael
Oxford University Press, 2009
Buku Teks SO
Bozeman, Barry
St. Martin's Press, 1979
Buku Teks
Helen Margetts, Perri 6, Christopher Hood, editors
Oxford University Press, 2012
Buku Teks SO
Weimer, David L.
Prentice-Hall, 1992
Buku Teks SO
Weimer, David L.
Pearson Prentice Hall, 1989
Buku Teks SO