Iowa State University Press , 1964
Buku Teks
International labour office , 1988
Buku Teks
Strong, Bryan
Wadsworth, 2008
Buku Teks
Manzer, Ronald A.
McGraw-Hill Ryerson, 1974
Buku Teks
McInnis, Edgar
Holt, Rinehart and Winston of Canada, 1982
Buku Teks