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.linuxdoc.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.
Optional - http://aspell.sourceforge.net/
This spell checking application can work around SGML tags, and only spell check the content within the tags. Older version of ispell will try to spell check the tags, causing errors at every new tag.
Optional - http://www.cs.hmc.edu/~geoff/ispell.html
The ispell program is distributed with RedHat (and possibly other distros) and also ignores markup tags.