Hey ho,
you could use the buildin Calibre Server. =)
https://manual.calibre-ebook.com/de/server.html
You start the Calibre Server in Calibre over the Button "Connect/Share". Now you can access over your WLAN the IP address, which Calibre will give you. You can find the IP Address, if you click on the "Connect/Share" Button again.
Put the IP address (
http://192.***.***.**:8080) into your Browser. Choose your Library and open it. On the top Menue is a Plus Icon. Click this! Now you can selecting the Book files from your phone.
That's all =)
Downside: It only works out of the Box in your WLAN. (Mobile Phone in the same Network) .
For more you would have to do more setup.