Update **5:
redsn0w version b5 incorporates yet another fix for iBooks, this time involving DRM. *@planetbeing wrote a utility called “crazeles” that overcomes jailbreak detection by iBooks that would cause about 10% of images to show incorrectly. *This fix is similar to the “hunnypot” fix that @comex wrote for the 4.x jailbreak. *As usual, you can choose to install the fix either by re-running redsn0w over your existing jailbreak (de-select Cydia if you do that), or by installing the corona package from Cydia (it’s the same set of files no matter which way you choose).
Here are the redsn0w download links: