Reynolds, John I.
International Labour Office, 1980
Buku Teks
Vonk, Roland, author
Prentice-Hall, 1990
Buku Teks
Millett, John D., author
McGraw-Hill, 1954
Buku Teks
Ohara, John B.
N,Y. Holt , 1964
Buku Teks
Adair, John, author
Pan Books, 1986
Buku Teks