So in this post we are trying to fix iBooks configuration error on iOS 5.1 as the result the iBooks will be working as it was before your device being jailbroken. And let's fix it.
How to fix iBooks error on iOS 5.1 iPhone and iPad
Step 2. Now open App store, download iBooks again
Step 3. Launch iFile (it's a jailbroken app available in Cydia) and head to: /var/mobile/applications/iBooks.
Step 4. Hold your finger on iBooks.app, keep it until you see a short menu appears and then tap on select!
Step 5. Now click Edit, then choose Copy/Link via copy board icon you see from the bottom right hand corner.
Step 6. Now, you need to go to root "/" to choose or tap on applications.
Step 7. Hold your finger down, wait till an option to paste appears. Then paste the folder to this directory.
Step 8. Go inside the new folder: iBooks.app to choose info.plist.
Step 9. Now choose "property list viewer" and continue by selecting "CFBundleIdentifier ".
Step 10. Now it's time to make a little change. Switch "com.apple.iBooks" into "com.apple.iBooksFix".
Step 11. Exit from iFile now, continued with respring your iOS devices.
Step 12. Again your rounded iBooks icon, once you've done than. Then respring your device again.
Step 13. Congratulation! You have fixed the iBooks Configuration Error on iOS 5.1. The app should be working fine now!