Updated to IOS 9.1 and now iBooks takes up ALL my storage

My 64gb iPad 4 (the first Retina one) was updated recently to IOS 9.1 and soon afterwards I got a message saying that my storage was nearly full so thinking little of it I deleted a load of games and apps I no longer use which freed up a decent chunk of storage but noted that iBooks was taking up around 13gb of space although I didn't think much of it at the time.

I seem to remember thinking there was 10-11 gb freed up by deleting apps and games I wasn't using.

Not long after I got the same message again, so I went back into my apps to see what was causing the problem and found iBooks was now taking up nearly 24gb of space... I had a few books on there that took up very little space (iBooks is under 2gb on my iPhone 6 and it has the same books in it) before I updated to IOS 9.1 but I deleted these books just incase it was something to do with that. This seemed to free a little storage but then soon after that I got the same message yet again. Now iBooks has taken up every single byte of free space on my iPad and I don't know why. Is there something that's broken iBooks in IOS 9.1? I'd noted that everything seemed to be running more slowly after I updated to IOS 9.1 if that helps (suggesting to me that it was busy downloading something in the background possibly?)

Any thoughts? Thanks in advance.