iBooks now allows iCloud syncing that mostly works (more to come on this subject, I'm sure). Maybe. Or maybe I just haven't used it enough to know how bad it is. Today I tried to download on of the iBooks synced via iCloud. It appeared to download but then when I clicked to open and read the book, I was shown this error message. I clicked "Settings" because I know this is wrong. I know I have enough available space for this 40-page books.
Or maybe this book is 20GB? (It is not.) Or maybe iPad requires 20GB of available space at all times? (Then "available" doesn't mean what I think it means.) Or maybe it is yet another bug in iOS, like the one that forced me to reboot twice just to get iMessage working after updating to iOS 9.3.1 yesterday.