Some users are experiencing issues getting iBooks to work after jailbreaking their iDevice from 4.2.1 (especially those who jailbreaked using Greenpois0n. Unlike the other fixes on the internet you don't need any files or such, all you need is good old Cydia
1. Go into Cydia and add " http://repo.insanelyi.com " as a repository in Cydia
2. Download "iBooksFix" which is free and install it
and voila! you have access to iBooks again on your jailbroken iDevice.
If the above method doesn't work you can always try the method in the below Video