Apple has updated one of the great books on Object Oriented Programming. This book has been cited in Java books by Sun (or so I've been told) as one to read to learn OO.

Object-Oriented Programming and the Objective-C Language (HTML)

or get the PDF version

Objective-C is a very fun language to develop in. I can't understand the stigma it's received. I learned Objective-C in a couple of days, on my own, by reading this book. It's a superset of ANSI C. It was EASY to learn.

Check it out!