This will only be a short review as I haven't actually finished the book yet!
- If you're a newbie to AS3: Buy it - If you're an intermediate developer: Buy it - If you've used AS2 before: Buy it
In short, it really is essential. Don't let the subject matter put you off. This book explains not only the how, but the why - something which many books with a similar aim often fail to do.
For example, I never realised there were two modes for the AS3 compiler. I had encountered but never used the [...rest] parameter, as it hadn't been explained properly.
This book will fill in the gaps in your knowledge. It covers AS3 completely. The only thing missing here is some really good workarounds for the Flash Player 9 memory leak issue, but that particular bug was not fully appreciated when this book was written.