For XML-based book standards,
Text Encoding Initiative's working on it. Also
DocBook, which I see you've found while I was previewing my post. Several tools available for generating HTML/ePubs from both, too.
On Google Code there's
epub-tools from ThreePress, who've a
nice ePub blog here. They love to show off nifty proposed stuff that will presumably be available in future versions of ePub. Their toolset includes the python-based tei2epub and docbook2epub, and some Java-based stuff.
There's also
tei2html which is a work in progress, but is also the
project of a fellow MR member.
Hope this helps.