Roberts, Don D.
Mouton, 1973
Buku Teks SO
Gallie, W.B.
Middlesex, Penguin Books, 1952
Buku Teks SO
Schwartz, Robert, 1940-
Wiley Blackwell, 2015
Buku Teks SO
James, William
Penguin Books, Harmondsworth, Middlesex , 1950
Buku Teks SO
Symons, Julian
Arthur Barker, 1951
Buku Teks SO