iBooks frozen

Have you tried closing the iBooks app via the iPad’s multitasking bar and seeing if it works when you re-open the app ?

iOS 8 : double-click the home button to open the taskbar, and then swipe or drag the iBooks app's screen from there up and off the top of the screen to close it, and click the home button to close the taskbar.

iOS 7 and below : from the home screen (i.e. not with iBooks '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 fix it then you could try a soft-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.