I seem to have found a way to restore an old version of iBooks - if you use a Mac with Time Machine.
The trick is to replace the file iBooks 1.5.ipa in the folder
<home./Music/iTunes/Mobile Applications
with an earlier version.
1. Make a backup copy of iBooks 1.5.ipa
2. Make the above folder active in Finder
3. Start Time Machine and go back until the iBooks file changes to an earlier version
4. Restore that file to the current folder
5. Start iTunes and go to the Apps section
6. Delete iBooks
7. In Finder move the iBooks 1.5.ipa file to trash
8. Drag the earlier version (eg iBooks 1.3.2.ipa) to the Apps window in iTunes (it does not automaticlaly recognise it)
9. In iTunes right-click on the iBooks app, select Get Info and make sure it is the earlier version
10. Connect your iPad and uncheck iBooks in its apps list then click Apply (may be this should be step 1?)
If iTunes asks you to transfer a purchase from the iPad click Dont Transfer as it is likely to be the more recent version of Ibooks
11. Check iBooks in the iPad's app list and click Apply. The old version of iBooks should install.
There does not seem to be a way to check version numbers on the iPad but the file size matches that of the previous version. Note that very old versions might have data compatibility problems.
Until Apple fixes the problem you should decline requests to update the iBooks app.