Saurik has released the updated version of Corona to 1.0-5 to fix the iBooks crashing issue in iPhone 4 and iPad running iOS 5.0.1 untethered. Additionally, Corona 1.0.5 untether also fixes launchd error while installing and downloading Corona on all A4 devices.
To install iBooks, you can just install Corona 1.0-5, which is available as a Cydia package and follow the step-by-step instructions here. Then simply, install iBooks from App Store and it should be working fine. Those users, who have already installed xBooks tweak from cydia for running iBooks must remove it, install Corona 1.0-5 and then reinstall iBooks again.
xBooks tweak developer @xvolks has been involved in the development of the patch and the patch is tested by @iH8sn0w. An initial update to version 1.0.4 was first released today in Cydia, but soon after Saurik added a force reboot on upgrade and pushed a new version taking Corona to the current 1.0.5 release.
Apple already patches the bug used in Corona in iOS 5.1 beta, so users with A4 devices, still not untethered should now upgrade to iOS 5.0.1 and untether their device with Redsn0w or Corona before iOS 5.1 update.