Garrison, D. R. (D. Randy), 1945-
E-learning in the 21st century : a community of inquiry framework for research and practice
Routledge, 2017
 Buku Teks
Town planning into the 21st century
edited by Andrew Blowers, Bob Evans
Routledge, 1997
 Buku Teks SO
The 21st century world atlas
Trident Press, [Date of publication not identified]
 Buku Referensi
Ward, Ralph D.
21st century corporate board
John Wiley & Sons, 1997
 Buku Teks SO
Yablonsky, Lewis
Juvenile delinquency into the 21st century
Wadsworth, 2000
 Buku Teks