You should be able to view the books offline - I suspect the password prompt is only appearing because you can also purchase books from within the iBooks application, the same way on the iPhone the password prompt appears when you open the App Store application.
If you're worried about the app not working when you have no Wi-Fi access, disconnect from your Wi-Fi network when you're at home, and open the application. You should still be able to view the books.
Try this I just read on another similar support thread.
Open up itunes app on your devices, iPhone, iPad, etc and check if you have pending downloads.
I checked both iPhone and iPad and sure enough there was "pending" downloads in the iTunes app. I cleared them and signed on to iBooks without having to re-enter password. Still a "Bug" in my book but at least a workable solution.