Core Animation is one of the coolest technologies to come out of Cupertino in many years. It makes doing all sorts of cool things in your application not just possible, but downright easy. Core Animation opens up worlds of possibilities for your applications' user interface.
The problem is, Core Animation is not intuitive. Even an experienced programmer can expect to spend a lot of time with Apple's documentation before he or she will grok the terminology and concepts of this new framework well enough to use be proficient.
Fortunately, the author has already gone through the trouble of doing that and has taken the time to break it all down into digestible chunks, explaining the concepts and terms in plain English, and setting out exercises that reinforce your understanding of those concepts.
As previous reviewers have mentioned, the bulk of the book focuses on using Core Animation when writing Mac programs, however both the basic concepts and the practical use of Core Animation are the same when programming the iPhone. The iPhone chapter explains the differences between using it on the two platforms and shows how to apply the stuff you've already learned earlier in the book when developing on the iphone.
I think this book probably reduced the amount of time I needed to learn Core Animation by at least half, probably more, and I'm an experienced programmer who's accustomed to learning from Apple's documentation.
Just a quick note of something that's probably obvious: You should already have a decent grasp on Objective-C and Cocoa before picking up this book if you want to get the most out of it.