Please try the following:
First thing to do is to force quit the iBook app on the iPad. Force quit the app.
— Press and hold the Sleep/Wake button down until you see the red “power-off“ slider—but don’t slide this time. Instead, try pressing and holding the Home button for about five to six seconds, then release. This should force quit the ibooks and take you back to the Home screen.–
Next step is to switch off your Wi-Fi and then switch it back again. For those of you that have both cellular and wI-FI on your iPad, toggle between the two.
Try opening your iBooks and check if you are able to access the store..
The ibooks store status can be found here https://www.apple.com/support/systemstatus/ . It seems to be working