iBooks hasn't worked since last iOS 9 update...why?

Without any information it's impossible to help. What do you mean by 'hasn't worked' (does the app not open, you can't open books in the app, books don't download from the store, ... ?), and what have you tried to see if it fixes it (e.g. closing the app via the phone's taskbar, soft-reset/reboot of the phone, backing up and copying purchases over to your computer's iTunes library and restoring to the backup) ?

I'm still not clear what the problem with the app is, is it that you can't actually open the app at all ? Or that it opens but then amends, or you can't open or download books in it ? As well as trying what I suggested, you've also tried copying your content off the iPad, resetting the iPad back to factory defaults, and just syncing content back to it (not restoring to a backup) ?

I Have the same problem. I go into books on my iPad it loads up then kicks me out after about a minute.I can read my kindle books and kobo but not iBooks on my iPad although I can read them on my newly acquired iPhone ( with a magnifying glass) tried reboot,recync whatever it's called....another afternoon wasted as it made no differance except that I had to look up all my passwords

i gather is a problem with a lot of people's iBooks.

I Solved it in the previous " update " by turning into airplane mode or whatever but then they decided to do another upgrade to prevent even that!

Same kind of problem. On my last iPad mini2 I could plug i in to itunes on a Macbook Pro osx10.9. I could click on On My Device in iTunes, click on Books and then drag any pdf in to that Books list and in seconds it showed up in iBooks on the iPad. I've lost that functionality with 9.3.1 on a new iPad mini. I was able to move about 5 PDFs through the laborious method of moving the PDFs in to iBooks on the laptop then syncing the ipad with the laptop, then the PDFs would show up in ibooks on the ipad. That only worked the one time. I can sometimes get the PDFs to show up in Books in On My Device but they have circles in front of them which mean ????? When I try to sync I get a message: cannot sync x items, refer to itunes. I tunes then tells me nothing about anything.

I just spent six months assembling a reference library for a dedicated iPad to be used in the field. Unfortunately I left that ipad on a plane with about zip chance of recovery and now in replacing the ipad I find it is not even possible to move copies of pdfs on to the new ipad.

Excuse the sarcasm but I assume I have to buy something now from itunes to make it work again.