You probably should not expect a fix until 5.1 gets untethered. The ibooks problem reportedly has something to do with how the kernel is jailbroken, on 5.0.1 the problem was solved in the package patching the kernel at boot-time to untether it (the Corona untether package). And it was apparently difficult to fix, so they probably concentrate on untethering 5.1 right now.
Have you tried the newest Redsn0w (Rerun latest redsn0w without anything checked, that will apply ibook fix)
I used 0.9.10b8b.
UPDATE: I downloaded iBooks and when I start it I get 2 Configuration Error messages and it quits.