I'll second that recommendation. It's great for getting up to speed on Cocoa and Objective-C very quickly. Combine that with the reference docs Apple provides, and it doesn't take long to start building decent OS X apps. :) The background knowledge you gain in how Cocoa works "under the hood" can be extended to other areas, as well - even writing simple graphical programs with AppleScript.

A word of caution, however, if you are using Panther: The most recent edition of "Learning Cocoa with Objective-C" was written for Jaguar. That means it still is oriented towards Project Builder, and not the newer Xcode. Most of the Interface Builder stuff still applies, but you may have to do a little poking around to adapt the Project Builder instructions in the book. It shouldn't cause too much trouble, but it is something to be aware of.