About CJK, at first I think there need to be a proper support for vertical writing layout and support for turning pages from right-to-left[1].

Hyphen currently seems to be handling vertical writing quite fine (but it is using default sans-serif font, which is less than ideal.) However page turning currently seems to be strictly left-to-right.

One of few apps I've seen to be doing well is Kindle (but unfortunately not ePub) and Bookwalker (has their own file format, although also has partial support for reading ePub file). My current solution for reading Japanese ePub books is to throw them into Bookwalker… which is kinda like second-class citizen in the app.

There are few public domain Japanese novels available in ePub3 format if you wish to have some sample (http://aozora.wook.jp/ click on "ePUBダウンロード".)

This is perfect, thanks! I've added it to the issue tracker: https://github.com/hyphen-reader/hyphen/issues/9