I agree, Cocoa Programming for Mac OS X by Aaron Hillegass is a great book to learn Cocoa, and it covers Obejctive-c nicely, but if you really want to KNOW the language, I would recommend:
Programming in Objective-C 2.0: http://www.amazon.com/Programming-Objective-C-2-0-Developers-Library/dp/0321566157
Both are fantastic books and nicely complement eachother.
Its very important to remember that other people are having a hard time learning Cocoa too. Reading Aaron's book is a fantastic way to start. Reading source code is also another great way. THere is some open source iPhone projects (for example: wordpress), and there a bunch for OS X. Read what other people are doing so you can get the flow of an application. I found that to be the hardest part about learning (especially the Cocoa part).