Hudson, Richard
Basic Blackwell , 1984
Buku Teks SO
Macmillan, 1981
Buku Referensi
Laski, Harold J.
George Allen and Unwin, 1948
Buku Teks SO
Laski, Harold J.
George Allen and Unwin, 1950
Buku Teks SO
Laski, Harold J.
George Allen and Unwin, 1955
Buku Teks