I wanted to continue reading a book on my iMac which I started to read on my iPad. This is what I got in return.
It says: “This book cannot be opened. iTunes shows error 42408. Reinstall iTunes and try again. Cancel/Load iTunes”
As far as I am concerned there is no iTunes any more. I am running macOS 10.15.5
You might try restarting your Mac. Issues with iTunes and iCloud syncing I find fixed by logging out of those accounts and then logging back in.
That could have been it. I logged out from my account from within Books. Then - surprise, surprise - a double-click on the respective book opened same. After logging back in I found the book-marks I made while reading the book on my iPad.
So problem = solved questions = remain