Apple restricted the iBooks service to open digital books loaded with DRM if the device was jailbroken. Comex has identified the cause of this error, and discovered a solution for it. And now, also, both the PwnageTool 4.2 and Sn0wbreeze 2.2 have been updated to bring the fix for this issue.
The name of this Cydia fix is iBooks Fix. In order to install this fix, all you have to do is, add http://repo.insanelyi.com in Cydia, and then install iBooks Fix.
After getting installed, all your DRM filled books should once again be accessible right from the iBooks app on your jailbroken iOS device.