Weird. I just checked again and all of my books (which is many 100's and I think about 4 gb space) are stable and not playing the "now it's here, now it's not" game. That's been for a least a week and it was completely reliable in removing them within hours prior.
That said, the space under "iPad Storage" says I have about 40gb free (when it used to say I had about 5!) and looking at per application space, it says iBooks is taking about 39.2mb (when it used to read 4 gb in iOS 10.x)!
They've done something weird with the storage. Not sure why I'm lucky enough to be stable, but it's definitely not doing what it was doing before for me. That said at one point I totally deleted and reinstalled iBooks )(though after that it was still broken_. Maybe that reset interacted with later versions to make it stable. It's been fixed for me since about 11.1 beta 2 (I meant to post it here earlier, but couldn't find the old email links).
Anyway, I'm about to go to 11.2 beta - we'll see if it changes anything. BTW anyone can join the beta program:
Apple Beta Software Program
I've generally found it has been safe, but caveat emptor, your mileage may vary.