Just to set the matter straight: not all common e-book formats are HTML-family-based. Popular exceptions are PDF and PostScript. ePub format is just a wrapper (albeit XML-based) for a number of different standard formats, not just those related to HTML; the RTF format that Kat suppressed is an example of a very common non-HTML format used in ePub books. (Ostensibly, any format you can describe by a MIME type can be wrapped as an ePub book, though in practice, only a limited number of formats are widely supported by popular reader applications.)

Although the trend in e-book formats is toward XML, providers like Amazon, Sony, Adobe and B&N are still intent on making them proprietary (restricted to their own reader devices--and unreliable support for older format versions), which is why I avoid proprietary formats for anything I might want to keep beyond the first read. Complicating the matter further are the un-unified Digital Rights Management schemes imposed to prevent piracy. Consider any "purchase" of a proprietary e-book to be a limited-term loan only.