2020 vision health in the 21st century
National Academy Press, 1996
 Buku Teks SO
Friis, Robert H.
Occupational health and safety for the 21st century
Jones &​ Bartlett Learning, 2016
 Buku Teks SO
Health care ethics: critical issues for the 21st century
edited by Eileen E. Morrison, Beth Furlong
Jones & Bartlett Learning, 2014
 Buku Teks SO
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