I would have to say that never having done any OO programming before in my life, I found _Learning_Python_ by Lutz & Ascher had a great couple of chapters on it. The diagrams about inheritance and subclassing really helped a lot and they describe the purpose of using OOP quite well. I see you already have that book, so perhaps you are not far enough into it yet to hit those chapters. I think they did a great job getting me off to a good start.
For a follow-up book, I second the recommendation of Mark Hammond's _Dive_Into_Python_, which is mentioned above. It's a wonderful way to learn by example. I absolutely ate up the chapters about Unit Testing. -- http://mail.python.org/mailman/listinfo/python-list
