“Cocoa Programming for Mac OS X” by Aaron Hillegass is generally considered the definitive Cocoa book. I haven’t read it, but everyone who has seems to love it. Of course, it focuses primarily on high-level UI coding in AppKit, of which there is little in Oolite, but it should cover Foundation (the lower-level part) as well.
Personally, I found Apple’s online documentation, particularly The Objective-C Programming Language
and the Cocoa Fundamentals
documents to be sufficient.
Since the GNUstep project has an explicit policy of attempting to track Mac OS X/Cocoa, thinking in terms of “OS X–specific Cocoa extensions” is probably not very helpful. The GNUstep people also point to Apple for documentation.