Marks, John
Heinemann, 1983
Buku Teks SO
Vasta, Ross
John Wiley & Sons, 1999
Buku Teks SO
edited by R.C. Olby, G.N. Cantor, J.R.R. Christie and M.J.S. Hodge
Routledge, 1990
Buku Referensi
Merken, Melvin
Saunders College Publishing, 1993
Buku Teks SO
Robertson, Barry C.
John Wiley & Sons, 1981
Buku Teks SO