Spaan, Ernst
University of Leiden, 1989
Buku Teks
International Labour Office
ILO (International Labour Organization), 2007
Buku Teks
King, J. E. (John Edward)
St. Martin's Press, 1990
Buku Teks
Anderson, Howard J., author
The Bereau of National Affairs, 1984
Buku Teks
King, J.E.
Macmillan press Ltd, 1972
Buku Teks