(There isn't really any reason to make a custom class Book in this case, as I didn't see anything other than accessors for it.. you could easily use NSMutableDictionary instead

you'd establish a binding from the array controller to the array key (on the File's Owner) rather than connecting it to the content outlet.

Hmmr.. looking at the article, and the source, I don't see where he's connecting the array and the array controller.