The new 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. RedSn0w 0.9.10b5 - 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).
If auto-detection fails and redsn0w tells you no identifying data was found, you can always pre-select the appropriate 5.0.1 IPSW using “Extras->Select IPSW”.