Become an Xcoder is a good guide to getting started (It is roughly equivalent to the simple random number application chapter(chapter 2 or so) of Aaron Hillegass’s book(which is fantastic, BTW).
1. MacDevCenter Contains some good (but somewhat outdated Cocoa programming articles)
2. CocoaDev Great reference site(code snippets and short explanations of various classes)
3. CocoaDevCentral Cocoa articles and blog entries by Mr. Scott Stevenson(somewhat intermediate level)
4. Apple’sCocoa Apple’s Cocoa site(all levels)
The one I know is good is Cocoa Programming for Mac OS X by Aaron Hillegass