Easiest way to scan ISBN number from books
Delicious Monster makes an awesome piece of software called Delicious Library that's intended to do exactly what you're after: scan and catalog your library (of books and other things). And it all works through your iMacs built-in iSight camera. And it works very, very well. From their website:


  “Wait, I just hold a CD or DVD or video game or book or whatever up to my webcam, and it magically reads the UPC and downloads that item’s cover and all pertinent information about it, and displays all my stuff on photorealistic shelves? I’ll take it! Right now! This is why I bought a computer in the first place! ”


Yea, it's very cool to watch it in action. Very easy to use.