Ive gotten a couple of questions lately from people asking what resources I use to learn Cocoa. I figured I would make a quick post, and list them:
Cocoa Programming for Mac(R) OS X (3rd Edition) - Aaron Hillegass
This is probably the best single resource if you want to just jump in and get started. It covers enough of the basics of Objective-C to get your going.
Programming in Objective-C - Stephen Kochan
This is a great resource for really learning the in's and outs' of Objective-C. Even if you are reason the Hillehass book, I recommend this one as a useful reference. (I am constantly looking stuff up in it.). There is also a new version coming out soon which covers Objective-C 2.0.
Apple Cocoa Reference Docs : Single best resource for API information.
CocoaDev Provides tips and tricks and gotchas for all of the Cocoa APIs.
Cocoa Dev Central : Good tutorial site that really compliments the Hillegass book.
Apple Cocoa-Dev Mailing List High volume mailing list hosted by Apple, which is the best resource for getting clarification on Cocoa, as well as help with issues you might run into.I have also posted my rough Cocoa notes, which you can find here. Finally, if you are considering doing any iPhone development, then make sure you learn how to manage memory when learning Cocoa. Objective-C 2.0 adds automatic garbage collection (which works well), but it doesn't work on the iPhone.