For information about the DocBook DTD, look here: » http://www.oasis-open.org/docbook/.
There is a DTD reference for DocBook at: » http://www.oreilly.com/catalog/docbook/chapter/book/docbook.html.
Get Going With DocBook, Notes for Hackers: » http://nis-www.lanl.gov/~rosalia/mydocs/docbook-intro.html.
You can also find more information and downloads of style sheets and other tools for DocBook in the DocBook Open Repository at: » http://docbook.sourceforge.net/.
You can also read the book » DocBook: The Definitive Guide online, or buy it at a bookstore, but be warned, that the first edition of the book (currently the only printed edition available) contains outdated information.
There is a great » DocBook Wiki, where you can collect information on DocBook editors, transformation tools, etc.