Palmer, Alan
Penguin Books, Harmondsworth, Middlesex, 1981
Buku Teks SO
Palmer, Alan
Penguin Books , 1987
Buku Referensi
Palmer, Alan
Penguin Books, 1981
Buku Referensi
Bannock, G.
Penguin Books, 1974
Buku Referensi
Thomas, Merlin
Penguin Books, 1985
Buku Referensi