Users of iBooks on jailbroken 5.0.1 devices may have been experiencing intermittent problems with the DRM associated with purchased book content. About 10-20% of the images in a book would fail to load due to a bug in the Corona untether.
The good news is that planetbeing corrected the bug in the Corona untether and issued an update
All jailbroken iPad owners should apply the fix by installing or updating the "Corona 5.0.1 Untether" package in Cydia to version 1.0-7 (or above).
This applies to everyone regardless of the jailbreak method you used for 5.0.1 (so yes, Absinthe iPad2 jailbreakers, this includes you!)
You will need to reboot once the package is installed or updated. After that your iBooks issues should be history [unless of course you are a dirty rotten pirate, in which case you don't deserve a working copy of iBooks!]