For many designers and developers who use Adobe Flash, the introduction of ActionScript 3 was met with some trepidation. The perceived increase in the complexity of ActionScript 3 code compared to ActionScript 2 - including the belief that you must use Object-Oriented Programming to use AS3 - has led some to decide to stick with AS2. This is unfortunate, as AS3 has a number of advantages over AS2. While AS3 is somewhat more complex than AS2, it is not prohibitively so, and the time required to bring oneself up to speed with AS3 is well rewarded.
"Learning ActionScript 3.0", by Rich Shupe and Zevan Rosser is, overall, a great introduction to AS3. The chapters are well organized, with a quick run-through of some familiar ActionScript concepts and code. If you've written any ActionScript before, you can skip this part, or skim through it just for some reassurance that not everything in AS3 is completely different from what you already know.
The subsequent chapters cover major aspects of ActionScript programming, ranging from graphics to sound and video to loading pretty much any sort of data. For example, the new display list in AS3 is thoroughly and clearly explained; as somebody still relatively new to AS3 I found this to be a pretty significant change to the way I think about Flash, so I appreciated how well the authors covered this part of AS3. And if you think that everything in AS3 only got more complicated, this book is worth it alone for the chapter on working with XML. These and other topics are explained clearly and thoroughly. The authors are both teachers at New York's School of Visual Arts, and their experience as educators shows through in their writing.
A neat aspect of the book is how it gradually transitions you into thinking about Object-Oriented Programming. For many people, the thought of having to do this with AS3 can be pretty scary. Initially, the code samples are meant to be placed right on the timeline. But part way in you get a primer on OOP. The book explains the concepts behind OOP very well, and gives the right amount of information - enough to get you going, but not so much that you'll get scared off at the thought of OOP. The code samples are no longer on the timeline, and suddenly you're working with object-oriented code, and it makes sense.
Those who are already familiar with AS2 will probably get the most out of "Learning ActionScript 3.0". If you're new to programming, and not just new to ActionScript, then this probably won't end up as the primary book you'll use to learn how to code. For example, topics that would normally get their own chapters in a 'learn how to code' book, such as variables, get only a section of a chapter by way of introduction. If you've already done some coding, this will be enough to reassure you that not everything is significantly different in AS3, but if you're new to programming, you might want something that spends a little more time on the basics.
In addition, the book would have benefitted from another round of proofing. There were a few typos that, although minor, were a bit of a distraction. The typos that I came across were pretty minor - nothing that you won't immediately notice, but it's too bad that they're there in the first place. And to be fair, I think I noticed only about a half-dozen throughout the book. (By the by, the errata page at learningactionscript3.com is much more complete than the page on O'Reilly's web site.)
Obviously, if the most significant criticism I can make of this book is that there were a few too many typos for my liking, I think it's a very good book. If you're looking to make the jump into ActionScript 3 coding, "Learning ActionScript 3.0" is a great place to start your education.