Instead of taking a book on Cocoa, if you're entirely new or even if you are not. You should learn the language that it's built upon. For instance, people think buying the books about Ruby on Rails will get them started, but Ruby on Rails is a framework for the web just like Cocoa is to the mac. It's Ruby that you want to learn, the language for the RoR Framework. As is Objective-C is the language for the Cocoa framework.
Sure you'll be able to follow the video tutorials or books on Cocoa, but you won't really understand what is going on or how it works. You'll be limited to the solutions the book or the video provides, whereas if you learn the language, you can do whatever you want.
Learn Obj-c then look how Cocoa works. Cocoa isn't really a language, it's a way of programming.