ABS is for audio books primarily (though they did introduce EPUB support). Unfortunately the API isn't standard so you can't just hack it into e.g. KOReader (though I have been working on a plugin for exactly that)
As far as I've seen, it's primarily designed for audiobooks, with ebooks just being tacked on. I'd advise to use a dedicated ebook service, e.g. Calibre.
I haven't seen any app, not even corporate-owned ones, come close to the awesome iBooks app (as I understand it Apple nerfed it so I'll say IOS 3 era iBooks). From what I've seen the ABS client is about as good as it gets, but I wish you luck in finding something even more polished.
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