"There is a problem with the configuration of your iPhone. Please restore with iTunes and reinstall iBooks."
Fix 1- Click ok then try to open the book again, keep doing this till it works. usually works after the second time for me, not the perfect solution but beats the others for me.
Fix 2- Remove iBooks from your iPhone ( iOS Device ) and then re-install it using iTunes don't Try doing so through the AppStore as it won't work. (problem may or may not re-occur.)
Fix 3- Do a Fresh restore for the firmware of the device then reinstall everything fresh don't use the restore backup option (problem may or may not re-occur.)
Till now there is no definitive pin point to the origin of the problem or whether it is Jailbreak related, or firmware version related (as I am still on 4.1 on my 3Gs)