Careless, J.M.S.
Canada : a story of challenge
Cambridge University Press, 1953
 Buku Teks SO
Careless, J.M.S.
Canada : a story of challenge
Macmillan, 1984
 Buku Teks SO
Mahathir bin Mohamad, 1925-
The challenge
Pelanduk Publications, 1986
 Buku Teks SO
Kouzes, James M.
The leadership challenge
Jossey-Bass, 2002
 Buku Teks SO
Myrdal, Gunnar, 1898-
Challenge to affluence
Pantheon Books, 1963
 Buku Teks