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Go to Featured in the iBooks app, scroll down and make sure your Apple ID is there. If it isn't, sign in.
If it is, try opening a random website in Safari, to test your Wi-Fi connection. A slow connection, or no connection at all, can be the reson for your problem.
If everything is working as expected, force close the iBooks app: double tap the Home button to show the App Switcher, then slide the thumbnail towards the top of the screen.
Hope that helps.