Harper , 1967
Buku Teks
[publisher not identified], [date of publication not identified]
Buku Referensi
Poldinger, Walter
Editiones Roche, 1984
Buku Teks
Bagehot, Walter
Oxford University Press, 1977
Buku Teks
Lord, Walter
Harper & Row, Publisher, 1967
Buku Teks