Delete the app and, if you haven't updated on your computer, resync the one in your computer's iTunes library. If you have updated the one on your computer, look in your Recycle/Trash Bin for the older app's .ipa file. Once you find that, delete the newer one from your iTunes library and put the old one back, and then sync it to your phone again.
(And then don't update.)
(Edit: if you have trouble with other versions, this may help: Fix iBooks Issue On iOS 5 Jailbreak Devices I've never used iBooks so can't give any firsthand reports.)