[tomsem]

Please explain what you mean by ‘downloading my books via Calibre’.

If I can assume calibre is running on a Mac, then the easiest way to make them available in Books is to simply open them using the Books app for Mac. If iCloud is enabled for Books, then they will upload to iCloud. But they won’t automatically download to your iPad: you need to tap on the ones with the ‘cloud’ icon on them when connected to the internet to download them.

(There is an option in Settings for Books app to ‘Automatically download purchases’, but as you are not purchasing these, I think that may not work with these books as they are not purchased from Apple. But I’ve never actually tried it.)

Once you download them to the iPad they’ll stay there until you Remove them.

You can also connect iPad to the Mac with USB cable, find the Books storage and drag epub/PDF to there.

You can also transfer them wirelessly using AirDrop, but you need a WiFi network connecting both Mac and iPad for that to work (do you have one?).