If you have read the earlier book, "Learning Flex 3 by Alaric Cole", you pretty much have an idea of the quality of work. This book lives up to that name and offers some delightful upgrades.
For a beginner, the world of Flex is very daunting and confusing. If you want a perfect book that assumes very little about your Flex background, this book lays the foundation for you to grow into a great Flex developer.
After you have been through the first 17 chapters and have mastered the contents, naturally the question arises as to where to go from here. Chapter 18 addresses exactly that. I was delighted to read quite a few new things like the "AlivePDF" library for generating client-side PDF files, the Facebook API, Google Map integration, etc.
The only drawback I found is perhaps a lack of mention on how to integrate Flex with a Java backend but there are plenty of online resources and O'Reilly books to help with that.