[cerement]

Mainly because I realize we are still in the "eBabel" stage of eBook formats, I'm wondering which format, so far, has proven to be most reliable and flexible when being used as a master or archival format for generating or being transformed into the many output formats (PDF, ePub, mobi, lit, lrf, etc., etc., etc.)? (And yes, I understand, your choice of output formats is still determined by what tools are actually available.)

XML Based:

ePub seems to be the dominant choice for a output format, but it doesn't feel quite rigorous enough yet for the master format (even in something as simple as confusion over using <q> vs " for quotes (related DTBook solves this the wrong way, by using both) -- but then, this is simply a matter of giving it time to mature).

TEI looks interesting and has been around for awhile, has the maturity, and has a couple useful implementations. TEI-Lite being the most well known. At one point, Project Gutenberg was working on PGTEI, but the status of that seems to be unknown at this point although documentation is clear and easy to understand.

Then there's the father (DocBook) and the grandfather (SGML) formats ...

TeX Based:

Mainly the LaTeX macros (and I love Donald Knuth's writing style and rigorous attention to detail). And while TeX can be output to almost anything, the conversion process between TeX and XML seems to still be pretty vague (although its ability to generate table-of-contents from full files would be very useful for the multi-file format of ePub).