Although it discusses Cocoa as of 10.2, this book is still a useful reference of the Foundation and AppKit frameworks in Cocoa.
The book begins with an overview of Objective-C and Cocoa's object classes. Additional topics include drawing and imaging, text handling, networking (sockets, network services, URL services), inter-application communication (distributed objects), and some additional non-Cocoa frameworks. These topics really are discussed "in a nutshell", but should provide enough grounding to figure out where to look for additional help. The bulk of the book is a reference of the Foundation and AppKit framework APIs; each class includes a summary, an inheritance hierarchy, and a listing of class and instance methods. The book includes not only a standard index, but also a method index which associates method selectors with the class in which they are defined.
This comprehensive reference provides developers who may be experienced with other application toolkits the grounding they'll need to start developing Cocoa applications. A complement to Apple's documentation, this book is the only printed reference to the classes, functions, types, constants, protocols, and methods that make up Cocoa's primary frameworks.
NOTE: This is not an beginning Cocoa book, as it is very dense and fast-paced. It will be of greater use if you've already got the hang of Cocoa and you're looking to broaden your knowledge about Cocoa, including the cool, rarely-used classes.