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.
A (proprietary) tool that will transform HTML documents into DocBook XML/SGML.