One of the known issues of the untethered jailbreak released for iPhone 4, iPhone 3GS, iPad 1, iPod touch 4G and iPod touch 3G is that it breaks iBooks, which is quite annoying if you use iBooks quite a lot on your iOS devices.
When you try to open iBooks on a jailbroken iOS 5.0.1 device, you get the following error message:
Configuration error:
There is a problem with the configuration of your iPod touch. Please restore with iTunes or redownload iBooks.
The good news is that iOS hacker with the Twitter handle xvolks has figured out a way to fix the iBooks issue.

He has also provided the following details on his blog:
With the invaluable help of @pod2g, I've patched the kernel in corona to make iBooks work without the ugly tweak called xBooks. Still no ETA. I've developed this patch on my iPhone 4. It needs to be adjusted to the other devices and being tested. Since I do not own an iPad nor an iPod, I cannot do that myself. Hopefully, MuscleNerd will do the rest of the job and will release the package soon. For xBooks users: When this fix will be released, the xBooks fix will be obsolete and you'll need to remove it and reinstall iBooks.
Since it looks like it will be released an update for the Corona iOS 5.0.1 Untether jailbreak, users will be able to fix the issue simply by installing the update.
Let's hope it is also fixed in the untethered iOS 5.0.1 jailbreak for iPhone 4S and iPad 2, which according to pod2G will be released soon.
We’ll let you know when a new version of Corona Untether jailbreak is released to fix the iBooks issue.
[xvolks blog via @pod2g]
Kindle app ftw
will it work on my iphone ive jailbroken thru redsnow? been waiting for that to fix my ibook…?
Yes it will work.
two thumbs up for teamwork