As a web developer, I was a little worried about picking up a compiled language to add to my toolbelt, since the paradigm is so much different from the normal scripting and object oriented programming I'm used to, but so far "Cocoa Programming: A quick start guide for developers" has been an excellent introduction. I am a big fan of PragProg books in general and this book is no exception. Written in a clear and conversation tone, this book is helping me get up to speed with the Cocoa framework from the very practical point-of-view of the XCode/InterfaceBuilder workflow and introducing Objective-C concepts in a way that is illuminating and not intimidating.
Its not a deep resource manual, there are other books that do that, but if you're looking to see what is like to build OS X applications, this is a great first step.