To make more storage available, iOS can remove some of your items, like streamed music and videos, files in iCloud Drive, and parts of apps that aren't needed. It also removes temporary files and clears the cache on your device. iOS only removes items that can be downloaded again or t...
There is a way, but it is not particularly user friendly and you have to be comfortable rolling up your sleeves and reaching into the guts of an epub file. Start with a .epub file that is not in the iBooks library. Unzip it. (I use "ePub Zip/Unzip 2.1.1") Save the artwork you...