24Symbols is a fairly new startup with a novel ebookstore idea. You can read online for free (and be subjected to adverts), or you can subscribe and read without ads and offline. Unfortunately, the current content selection is almost entirely in the public domain. There's nothing on 24Symbols that you can't find on Project Gutenberg, Feedbooks, or other sites.
But I did notice one interesting detail. Like the apps from the major ebookstores. the 24Symbols app fell victim to the iTunes rules about linking to the website. You have to have an account before launching the app otherwise you can't do squat.