Sibley, Rob
Stanley Thornes , 1994
Buku Teks
Simie, Andelko, author
2005
Artikel Jurnal
Laski, Harold J., author
George Allen and Unwin, 1947
Buku Teks
Laski, Harold J., author
George Allen and Unwin, 1936
Buku Teks
edited by Anthony Thorlby
Penguin Books, 1969
Buku Referensi