The more your online content diverges from a traditional Web page, the less likely a format that isn’t HTML is going to work. The less your content resembles a Web site, the greater the need to use HTML, at least if you intend to distribute it online. Any format that is not HTML in some guise has a short life expectancy.
Electronic books are not Web sites. You can post your book copy as Web pages, but the E-book as a logical entity is not a Web site. ePub, the international E-book standard, is HTML (XHTML 1.1 with minor exclusions). Every device under the sun except the Kindle can display your ePub electronic books. (A Kindle can show you its own variant, .AZW, of a variant of HTML [Mobipocket]; that’s two steps removed from the real thing. It can also convert HTML to something else. Actually, the Kindle encourages incorrect thought more efficiently even than Windows: Markup is considered “formatting,” which it isn’t.)
If you’re blind, the highly preferred E-book format, DAISY, is also XHTML.
What might pound the last nail in the coffin for non-HTML formats? Well, the iPad, obviously. Its iBooks (sic) are all ePub, which “is like number portability in the cellphone world.”
HTML does not work for all documents, since it lacks important structural features. It works for huge numbers of documents, many of which we call books. Bet against HTML for online distribution and you’ve backed the wrong horse.