Well folks if you dont want to switch from iBooks to KyBook 2 or Marvin then there is another option.
You can use another program to do what you used to do with iTunes. iMazing or SyncIOS or similar works fine.
Disclaimer: I have nothing to do with any of these products.
https://imazing.com/
https://drfone.wondershare.com/itune...to-itunes.html
https://beebom.com/best-itunes-alternatives/
You don't have to stop using iTunes, you just additionally use this for books.