Apple has attempted to prevent Jailbroken device users from reading books! Books people! Might as well have a good ol' burn burning.*
If you are Jailbroken and have updated to the newest version of iBooks, you might get a message, while trying to open a bought book, something like the following:
"There is a problem with the configuration of your (enter iDevice here). Please restore with iTunes and reinstall iBooks."
Nice try Apple, but you would get my Jailbreak off that easy. JAILBREAK AND NO READING FOR YOU!*
This happens when opening a purchased book because iBooks runs an unsigned code. Non-Jailbroken iDevices cannot run unsigned code but Jailbroken devices can. This means that when you open up that book and your device can run the code, it will open your book but then block you because it knows your Jailbroken.
The Chronic Dev Team is working on a fix so sit tight. For now, try redownloading the book you want to read.