BooksApp - Android Market
I downloaded and started using this app tonight. Essentially it has a barcode scanner that scans your books, looks up the cover and details on Google Books and adds it to your own personal library. You can then set books to out to loan and all sorts of other useful things. A great feature is the ability to email as a csv or upload to Google Docs.
I scanned my upstairs books this evening and out of just over 200 it got almost all of them. The main omissions were just a couple that didn't have barcodes because of being too old or not registered with isbn numbers.
I have about the same again to scan downstairs tomorrow before packing into boxes (courtesy of @GiantBassstacks).
I had the phone in a grip stand so I could just slide the books underneath so got them scanned in no time at all. A very geeky evening indeed!