Things have changed drastically with Catalina – this may change after the beta is done – hopefully for the better. To get your files into unrealBook or unrealBook Mobile, connected the iPhone or iPad to a Mac. Open a Finder window. (The files are now accessed directly in the Finder). Click on the iPhone/iPad icon on the left and click on Pair if it asks. You will need to unlock and “trust” the computer.
Then once the apps are listed under Files, you can click the disclosure button to show the files. Be patient as it can take a while. Drag files into the files area to add them to the iPhone/iPad. Unfortunately you cannot shift select files to delete them.