I’ve heard Kobo is better than the other big players when it comes to interoperability with open formats / self hosted setups.
As for the servers
The main one
https://github.com/janeczku/calibre-web
A popular newer one
https://github.com/crocodilestick/Calibre-Web-Automated
Also (to everyone else reading your comment) let’s not downvote good faith comments, especially when they’re phrased as a question and wanting to learn more
I should have specified, people we’re downvoting you
But looks like the score is positive again 🙂