iPhone: How to ensure iBook files are stored locally
Related question: Books automatically deletes my books when offline, and when there is enough space on device


  how to check reliably that files are stored locally on iPhone (ie not iCloud)? I mean, I could use the airplane mode and see if files are still openable ... but maybe there is a cache or something


If you are on iOS 12, then go to library and next to the book at the bottom-left there will be a cloud symbol if it is stored in iCloud and no cloud symbol if it isn’t.


  actually, how to reliably use only local files - and get rid of iCloud access, at least for iBooks [ iCloud is off on my iPhone for iBooks, but files disappeared anyway ]?


Got to Settings -> [your name] -> iCloud and make sure Books is turned off.

Note: make sure you download all your Books before turning it off.