Just finished ActionScript 3.0 With Design Patterns, recommended to me by a colleague. It introduces ActionScript 3.0 nicely, while pointing out the best practices and common pitfalls. It covers the more popular and useful design patterns and will change the way you code.
However, it had quite a few code typos – most are easy to spot and may be corrected in the next edition anyway – but you will need a sound knowledge of ActionScript already to find this book useful. Despite that, it’s well-structured and highly recommended.