So apparently when your iPad is getting low on space, it just deletes all the books from your iBooks library. Without asking for permission or notifying you in any way. It "offloads" them to the iCloud, so you have to re-download them manually.
Which is a very unpleasant thing to find out after you went on a summer vacation to the area without the internet, hoping to plug off, having downloaded a bunch of books you were hoping to read.
This is the most infuriating moment of using a piece of technology I've ever had.
I'm posting this here on the off chance that someone from Apple reads this, since that happens with threads on HN sometimes.
We’re about to board a long haul flight and our son was too eager to download Netflix kids programs so that space ran out. Apple has a default policy of offloading unused apps to conserve space. Unfortunately it offloaded the Netflix app itself :-/