Some users were complaining about newly released iBooks for iPhone having issues on jailbroken iOS 4 devices.
The good news is that Redsn0w has been updated to version 0.9.5b5-4 in order to fix this problem.
The official change log is as follows:
There’s a new redsn0w beta (links were changed above) that should fix any iBooks problems people were seeing. Just run this new version 0.9.5b5-4 and deselect Cydia (you don’t want to reinstall Cydia over itself).
Instructions to jailbreak iPhone 3G and iPod touch 2G on iOS 4 using Redsn0w 0.9.5 as usual are exactly similar to the last release. Please read all the instructions posted here carefully before upgrading and jailbreaking your phone on iOS 4.
Once you have jailbroken your phone, you can unlock it using ultrasn0w 0.93 (on any baseband), guide for which is posted here.