User can scan any item's (book, CD, DVD, etc.) ISBN or UPC barcode at, e.g., friend's house or bookstore to look up the item's availability in library and place a hold on it right away.

You can get to scanning a barcode from search screen:

Once user clicks the icon, a camera screen will open up. Once pointed at barcode, it will locate the ISBN or UPC, and open up item details in the app or will display a message that the item is not found.