Ward, Brice
Blue Ridge Summit TAB Books , 1971
Buku Teks SO
McCracken, Daniel D.
John Wiley & Sons, 1974
Buku Referensi
O`Donnell, Sandra Martin
Prentice-Hall, 1994
Buku Teks SO
Stein, Lincoln D.
Wiley, 1998
Buku Teks SO
Mann, Thomas
Oxford University Press, 1998
Buku Teks SO