For downloading, I use Readarr or manually download from where I buy the books. A lot of the time the books I buy have DRM so I end up pirating them anyway 🤷. When readarr sends a book to calibre, it tells it to convert it so I’ve always got epub and mobi available.

For accessing my library, I use calibre-server, which I think comes bundled with the calibre desktop app. It’s got a basic web interface for uploading, editing some metadata, and downloading; and an OPDS API which my e-reader can use to download books. If I’m outside my home network I use my VPN to access it because I don’t trust calibre to be secure enough for internet exposure lol.

Don’t recall why I chose this instead of calibre-web but it works fine for my purposes.