https://github.com/koreader/koreader/releases
If people are still looking for an alternative for calibre companion they can probably use koreader it has built in calibre wireless connection so you are able to transfer your files to your device wirelessly then you can use the any other reader you want to read the files
https://github.com/koreader/koreader...ess-connection
if you want some of the calibre companion features then you can use this koreadr plugin but it works best when you are using it as a reader as well
https://github.com/harmtemolder/koreader-calibre-plugin