A quick fix is now available in Cydia store that lest you read the DRMed / rightfully purchased iBooks on already jailbroken device on iOS 4.2.1.
Here is how to install iBooks Fix.
Step 1: Tap on Cydia icon and go to Manage -> Sources -> Edit -> Add.
Step 2: Type the address of Insanelyi repositr0y and tap on “Add Source”.
Step 3: Search for “iBooks Fix” and install it.
After installing the iBooks Fix you can read the purchased content from iBookstore including Winnie-the-Pooh.