There are lots of pages about DocBook out there.
The "official" homepage is http://www.oasis-open.org/docbook, the DocBook homepage at OASIS. You can download both the latest DocBook XML DTD and DocBook SGML DTD there.
The DocBookTheDefinitiveGuide website is http://docbook.org/; that's also where this Wiki is hosted, if you hadn't noticed
The DocBookOpenRepository is at http://docbook.sourceforge.net/.
The DocBook FAQ, maintained by the ever helpful DavePawson, is at http://www.dpawson.co.uk/docbook/
Website, the docbook based website publishing system, is at http://www.dpawson.co.uk/docbook/website/
The FreeBSD Documentation Project Primer at http://www.freebsd.org/doc/en_US.ISO8859-1/books/fdp-primer/ contains a section with information about a variety of DocBook elements, and includes a general SGML guide as well.
A collection of DocBook links can be found at the docbookmarks page: http://www.dulug.duke.edu/~mark/docbookmarks
Wikipedia, the free encyclopedia: http://www.wikipedia.org/wiki/DocBook
A Korean DocBook website: http://docbook.kldp.net
A German DocBook website: http://www.goshaky.com/docbook-tutorial/ - only XML
A Czech DocBook website: http://docbook.cz
Weblog on DocBook and other Topics http://weblogs.goshaky.com/weblogs/page/lars
DocBook Demystification HOWTO by Eric Raymond
Another German DocBook website: http://www.docbook.de/
The Russian DocBook website: http://docbook.ru/