Vaillant, George C.
Penguin Books, 1950
Buku Teks
United Nations Educational, Scientific and Cultural Organization, 1977
Buku Teks SO
Laski, Harold J.
George Allen and Unwin, 1936
Buku Teks SO
Laski, Harold J.
George Allen and Unwin, 1947
Buku Teks SO
Diggins, John Patrick
W.W Norton, 1992
Buku Teks SO