Comex discovered that the app was checking if the operating system allowed it to execute an unsigned binary, and if it did, the book wouldn’t open. The good news is that both PwnageTool 4.2 and Sn0wbreeze 2.2 have been updated to include a fix, but what if you’re already jailbroken using Redsn0w 0.9.6, 0.9.7 or GreenPois0n?
A quick fix is available on Cydia, that fixes iBooks.
How to Fix iBooks crash
Launch cydia and add http://repo.insanelyi.com repository and then install “iBooks Fix”. That’s all.