I'm running Kavita in parallel for my tiny Manga collection and I'm not really happy with it compared to Calibre-Web. The web-reader is better for mangas (that's why I'm using it) but everything else is worse in comparison.

For actual reading I am looking at Tachimanga with the Kavita extension, it looks like as long as the files are recognised by the server properly it reflects fine. You can also find extensions for other sites if you get your manga that way.

Kavita repo -

https://raw.githubusercontent.com/Kareadita/tach-extension/repo/index.min.json

Other repos -

https://raw.githubusercontent.com/keiyoushi/extensions/repo/index.min.json

https://raw.githubusercontent.com/yuzono/manga-repo/repo/index.min.json

https://raw.githubusercontent.com/Suwayomi/tachiyomi-extension/repo/index.min.json

what's the difference between Kybook and the default Books app?

I haven’t used the default books app in a long time but it was just lacking in customisation when I tried it. Kybook3 lets you customise a lot and manage your books differently. It also has the function to connect to OPDS so you can directly connect to calibre-web or any other OPDS server like Project Gutenberg or Internet Archive to download books.

Please note - KyBook3 has not been updated in years and the premium sync service does not seem to be maintained. Do NOT spend money on the subscription. I think the Pro works, I can’t remember what it unlocks but as long as they are local to the app and don’t require an external server it’s fine.