The Chronic Dev Team has just patched GreenPois0n for iOS 4.2.1, the update includes the iBooks fix. The issue was found recently (quick question: anyone else find it funny that it seemed to take a little while before anyone even noticed that iBooks didn't work properly?) and Comex released the fix the next day. The problem was that the user had to run the fix themselves, and it required some basic knowledge of connecting your iPhone though SSH and finding a particular folder, etc. Easy enough, but probably not for everybody.
Luckily, GreenPois0n RC6.1 now comes with the fix to make iBooks work properly again.
Here are the downloads:
We posted a 'How to' a while back. Since the jailbreak is the same other than the iBooks fix, you can go here.
If you already have your device jailbroken, but just want a quick fix, you can check out this guide, it's the same fix mentioned above, the difference being that you don't have to reinstall your custom firmware.
If you have an unlocked iPhone, you may be interested in heading here for PwnageTool 4.2, which will let you keep your current baseband and use UltraSnow to unlock. Know that it also includes that iBooks fix mentioned above: Here.