Walker, Gilbert
Heimann , 1957
Buku Teks
Clapham, John, Sir, author
The Cambridge University Press, 1949
Buku Teks
Alford, B.W.E., author
Macmillan, 1988
Buku Teks
Milward, Alan S.
Macmillan, 1984
Buku Teks
Scherer, F. M., author
Rand McNally , 1970
Buku Teks