That message happens when CC cannot find the book file where CC thinks it should be. That could happen if the book file is indeed gone, or it can happen if Android has decided that CC doesn't have the right permissions to read the book. You didn't mention whether or not the books are on an SD card. If so, then I suspect permissions.
If you send me a CC debug log (see
How do I get help? in our FAQ. Please erase the multipie.co.uk address.) then perhaps I can tell you what is happening.
Another thing you can try, especially if the books are on an SD card, is to tell CC again where the book files are stored. Go to Settings / Formats and folders and change the default folder to itself. You do this by tapping on the folder line then using the navigator to select the same folder. See the first part of
How do I use an SD card with Android 4.4 (Kitkat) or later? for more information.
If you have the patience to wait a bit, I prefer that you send me the debug log. That way I (hope I) can tell if the problem is on your device or something more global.