You probably already know about these:
First, go to the Apple site and download their free publication, "Object-Oriented Programming and the Objective-C Language." (Or you could buy it ... it's offered by that California bookseller that was just bought by Barnes & Noble -- can't remember their name -- website is "geek" something.) It's 1.5 MB or 194 pages.
There's the O'Reilly book, "Learning Cocoa". It isn't the best, but reviews say that chapter 3, "Objective-C primer" is good.
Last, and most highly rated is "Cocoa Programming for MacOS X" by Aaron Hillegass (www.bignerdranch.com). As far as I can see, it uses Objective-C exclusively.