After the recent untethered jailberak of iOS 4.2.1 iBooks with DRM have been giving an issue with jailbroken iPhone's giving the error

"There is a problem with the configuration of your iPhone. Please restore with iTunes and reinstall iBooks"

there is a fairly simple fix available for this which only requires the use of Cydia.

1. Go into Cydia and add " http://repo.insanelyi.com " as a repository in Cydia

2. Download "iBooksFix" which is free and install it

and if for some reason the above method doesn't work for you give this video a try

Source: I Heart Apple