Ever since the new iBooks update, people have had troubles opening their purchased books within the app. Well this is because you are Jailbroken and your device can read unsigned code. Every time you open an ebook, the device executes a code, and if that code can be executed it means your device is Jailbroken. This of course means that you will get a silly little pop up message basically saying "HEY! HEY! HEY! Unjailbreak your device RIGHT NOW MISTER!" So we don't want that happening anymore.
Do the following steps and you can get back to your reading:
Step 1: Type in Safari:
Step 2: Choose Open in "iFile" and once iFile is open, tap Installer. You will get a return code of 0 if it worked correctly.
Step 3: Exit iFile and respring your iPhone
Step 4: You're done!