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.