The items in this section are reference books or other utilities that can't quite be categorized (yet).
http://www.docbook.org/
This book was released by O'Reilly in October 1999, and is a great reference to DocBook. I have not found it to be a great practical book, and much of the emphasis is on XML, but the DocBook tags for version 3.1 are all listed in a handy format. You can pick it up at the book vendor of choice. The entire book is also available online (in HTML and SGML formats) at the above URL.
Optional - http://www.tldp.org/authors/index.html#resources
Contains links to SGML templates and their resulting HTML output to help you see what your document will look like. Many of the tags just need to be replaced with information unique to your HOWTO. Also contains links to tools and other useful information.