This is a thick, substantial book, very useful as a reference, even though much of the material is somewhat dated. It covers Cocoa as of 2002 and OS X 10.2 "Jaguar".
It is a comprehensive work that starts as a fast-paced introduction to the OS architecture and the Cocoa framework for programmers new to the environment. The more advanced sections of the book show the reader how to create Cocoa applications using Objective-C, modify the views, integrate multimedia, and access networks. The final sections explain how to extend system applications and development tools in order to create your own frameworks.