I don't think those are running the Calibre Content Server. I was hoping to find a live instance so that I could see what the books look like in my browser.
In Japanese or translated to English? If you are interested in a collection of Japanese bestsellers/classics, translated to English, then asking over at the /lit/ forum of 4chan (hear! hear!) will yield you some interesting answers. Other...I don't know.
[Izzy's eBook Library of English Public Domain books](https://ebooks.qumran.org/opds/?lang=en)
[Izzy's eBook Library of German Public Domain books](https://ebooks.qumran.org/opds/?lang=de)
[Feedbooks' have a "Free" category in this following library](http://www.feedbooks.com/catalog.atom)
[Free books on Beam Shop](https://www.beam-shop.de/angebote-specials/gratis/?p=1) (from a quick check, titles were in English, but could be German, too, or mixed)
[Manybooks](http://manybooks.net/opds/)
There are also others but in Hungarian, Russian, Japanese languages. Besides, there is always Gutenberg.org for Classics.