I'm really disappointed by the ebook software available for Maemo. FBreader is pretty lame, given its lack of support for CSS and its other shortcomings. And yet, it's the only game in town. Oh, if only iSilo would come out with a Maemo version!
I've seriously contemplated rolling my own. Epub is nothing more than XHTML and CSS with a little added metadata, wrapped in a ZIP file. Seems like it wouldn't be *too* much work to make a reader based on WebKit, or maybe XulRunner. (You could almost use an existing browser as-is; the biggest problem is that it doesn't remember your position on the page.) Unfortunately, I already have too many side projects going on.