DocBook is a DTD usable with SGML and XML that was designed for authoring technical documents such as computer manuals. A number of tools are growing up to assist in processing DocBook-based material.
This document describes how one might use the DocBook DTD to write documents, including information on how to use tables, and insert program listings.
Hopefully it will soon document the care and feeding of insertion of graphics, and more information on "interesting" crossreferencing.
My review of DocBook: The Definitive Guide as written for the DFW Unix User's Group.
FreeBSD Documentation Project Primer for New Contributors (SGMLTools)
A (proprietary) tool that will transform HTML documents into DocBook XML/SGML.
OOo2sDbk - Convertisseur OOo-Writer vers Docbook
Python-based software for converting LibreOffice word processor documents into DocBook .
I wrote translation code in Common Lisp that transforms the XML-based data format into DocBook form.