On February 26th, Kindle customers will lose the ability to download eBook purchases directly to their PC. If you want to switch to a rival eReader brand in the future, I suggest that you use the soon-to-be discontinued “Download and Transfer via USB” feature to archive your Kindle library.
There’s also calibre-web for a self-hosted option with a web interface.
IIRC it’s not as feature complete as the fat client
This. I’ve personally found it easiest to use Calibre to strip DRM & get things tidy, then use Audiobookshelf to manage both my ebooks and audiobooks
It does ebooks too?