A very important topic in my opinion, whether you are working on the Mac or iOS platform, is grand central dispatch, understanding the mechanics of ios concurrent queue marshalling and this book does the trick. It's short in size, but with iOS4 and Blocks, it makes working with something on the main thread, on a split thread, more comprehensible, and the author provides concrete examples on how to utilise them, but it's definitely not for the novice developers.
But for efficiently tweaking your code, for an intermediate developer, this book is a must, and I am a fan of Nahavandipoor, having read his previous book on Graphics. But I am happy with content level, I perhaps would keep this book as a reference and then dig deeper into it later, and even though I am primarily iOS (not Mac OSX) developer, it is good to understand how the principles work in iOS's cousin, the desktop OS.
Prior Knowledge: Objective-C, Mac OSX Development experience
My rating :[rating=3.5]
Author: Vandad Nahavandipoor
Publisher: O'Reilly Media
Year: May 2011