I have to add to the list, Peter Cooper's book, Beginning Ruby, is a  
pretty gentle intro as well.
Consider yourself lucky, Ruby makes most things objects already, so  
you don't need to immediately create any classes yourself.
You can just play around with the existing objects and see how they  
work and relate to eachother. irb is  great for this, because you can  
ask objects what they can do and who their parents are!

I have to disagree about Apple's OOP intro for Objective-C. I had a  
hard time with it. There is a good book on Objective-C from Stephen  
Kochan, and it gives a pretty decent OOP intro, but not as good as  
what's in the Ruby books out there.

I've found that Ruby made OOP a lot easier to understand and  
consequently makes other OOP languages make more sense to me.

Other OOP languages usually come a lot easier with a background in C   
(Objective-C , Java, C++, etc...)

In my humble opinion, if you learn Ruby and C, the others will come a  
lot easier!