First off, this book has received bad reviews for not being technical enough, but frankly that's not the scope of the book. If you're not familiar with Flex and looking to learn Flex Builder and the Flex API, this is not the correct book. It's sister from friends of Ed will serve you much better:
Foundation Flex for Developers: Data-Driven Applications with PHP, ASP.NET, ColdFusion, and LCDS
As a hybrid designer/developer with 7 years experience developing complex applications in Flash, I've been making the switch to Flex. I found that once I had a grip on the basics of Flex, I was left with a conundrum. "OK it works, but how do I make it pretty?" Coming from Flash where it's really easy to just draw a graphic and be done with it, Flex will seam a bit stark. Flex for Designers came to the rescue in my hour of need. It clearly explains how to style the Flex components with CSS, and various ways of using graphics generated in Flash, Fireworks, Photoshop & Illustrator.
If you're a designer working with a Flex developer to generate graphic resources, this book will show you multiple ways to do so, so you can choose the best techniques for your work flow. It will not, however, teach you how to make working applications in Flex.
Most of the information in the book is available elsewhere but often buried within other concepts or spread all over the net. This book is an indispensable guide for making the Flex components look the way you want them to, from simple style changing to completely custom graphcis.