Grayson, Leslie E.
John Wiley & Sons, 1981
Buku Teks SO
Grayson, James Huntley
New York : Routledge Curzon, 2005
Buku Teks
Oates, John
Blackwell , 2004
Buku Teks SO
Trulove, James Grayson
Massachusetts Rockport Publishers 2000,
Buku Referensi
Covington, Vicki
Penguin Books, 1990
Buku Teks SO