Why can't I access the bookshelf on I pad 4?

Is this in the iBooks app or a different app, and what happens when you try ?

What happens when you try to access it, does the Library button for returning from the store not work/respond ?

Try closing the iBooks app via the taskbar and see if it works when you re-open the app : from the home screen (i.e. not with the iBookd app 'open' on-screen) double-click the home button to bring up the taskbar, then press and hold any of the apps on the taskbar for a couple of seconds or so until they start shaking, then press the '-' in the top left of the iBooks app to close it, and touch any part of the screen above the taskbar so as to stop the shaking and close the taskbar.

If that doesn't work also do a reset : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content, it's the iPad equivalent of a reboot.