I think anybody who spends time formatting ebooks should take a serious look at the
Daisy DTBook XML element set (which is actually included as part of the epub spec).
It allows semantic markup (ie. you use tags like <author> and <epigraph> rather than presentation tags like <center> and <font>) and inclusion in epub means that hardware readers should support the format natively in the near future.
Perhaps best of all; documents that you spend time marking up in DTBook should be accessible to those with vision problems - via text-to-speech and braille converters.
CSS (and possibly XLST) used in combination with a DTBook document should allow a visual presentation almost as good as using HTML/CSS, but with the advantages of greater accessiblility and semantic clarity.
Also it appears to be a fairly small element set, so it doesn't have the steep learning curve associated with TEI. Marking up something with a simple structure like a novel should be very easy.