Why are we supporting .epub? Why not, for example, Microsoft’s .lit format, or PDF, or better yet, LaTEX?
The answer is: Web. The ePUB format is the most “webby” of all formats. Not that those .lit files of yours are not just a hop, skip and a breach of the DMCA away from being webbed-up books. After all, .lit is a kind of flatfooted ancestor to ePUB. Some would say it’s better suited to the publishing world because of its DRM attire.
But in evolutionary terms, that will become a mere nub on the anatomy of the ideal book format of the future. No, ePUB looks forward, toward CSS3 and the networked reading populace of the blogosphere. It sees “page” as a limitless concept and not a finite space. Documents in ePUB have reflowable text that can adapt to different screen sizes; like those little mammals that scurried through the brush, it adapts to the space it’s given, doesn’t crash through undergrowth like a big dumb lizard.
Oh, it has its drawbacks. Print purists will cringe at the limitations of CSS and HTML. Publishers will howl at the lack of DRM. Hypertext snobs will sneer at the “bookishness” of its assumptions. But developers will play with it. And apps will grow around it. And it will grow with the web itself, where more and more books are going to live.