I became interested in DocBook after seeing that Dive Into Python was available in so many formats.
From the Docbook Wiki
DocBook enables you to author and store document content in a presentation-neutral form that captures the logical structure of the content. Using free tools along with the DocBookStylesheets from the DocBookOpenRepository and related resources, you can transform, format, and publish your content as HTML pages and PDF files, and in many other formats, including Microsoft HTMLHelp, UNIX man pages, JavaHelp, TeX, TeXinfo, and RTF.
There's even an O'Reilly Docbook book with a picture of a ...duck on the front. O'Reilly's Docbook book is online too.
References: http://docbook.org/
Notes: I wonder how well DocBook is suited for Japanese text?
DocBook is cool, but to heavy. It's for whole books, not for everyday use. On Debian Linux for DocBook it need 27M for all software!
Try something lighter - for example Gentoo Linux XML format:
http://www.gentoo.org/doc/xml-guide.html