As part of my community involvement ([...] and [...] one of my goals is to have read every Flex book there is so that I can advise people when they're looking for a book which book is right for them. The amount of Flex 2 books out there is awesome, and we're definitely far away from being able to say there's "too many options". Currently I've read The Essential Guide to Flex 2 w/AS3, and am reading RIAs w/Adobe Flex and Java, and Programming Flex 2 - The Comprehensive Guide.
All of the books I've read so far have been great, and having 8 yrs of experience in the Knowledge Management industry, if the content is relevant and accurate, it's valuable. Each of the books have different approaches, breadth, and depth of subject matter. So I wouldn't say one book is better than another - more important is to find the book that best matches how you learn, where you're at learning curve wise, and how committed you are (do you want to just know enough to make some cool CRUDs, or become an extreme RIA ninja).
One of the things I've found with Flex is it's very difficult to talk purely about one subject without having to reference surrounding topics to make examples have some kind of context. E.g. it's hard to talk about Effects without mentioning Events. Do you cover all of ActionScript up front, or try to integrate AS understanding as you go along piggy backing on other subjects?
So there's two ways to tackle this - introduce a lot of things at once, and as the book goes along you progressively increase on depth; or topic by topic you lay it all down, so that once that topic is covered you can utilize that info in subsequent topics if you need to.
Both are valid approaches, and all I would say is consider what kind of learner you are. Do you like knowing a little about a lot upfront to get a big picture, or do you find that too overwhelming and just want very focused topics and know all that there is to know about that topic.
At the same time, if you wanted a lot of width and breadth, you'd end up with a 3000 page book. So look at the chapter listing of all the books you're considering and see how many topics they cover, if it's A LOT of subjects and roughly the same amount of pages of another book, you're going to get more breadth and depth. And vice versa, fewer topics over the same amount of pages is usually indicative of depth.
Recently I finished Professional Adobe Flex 2, by Rich Tretola, Simon Barber, and Renaun Erickson. I've actually had the pleasure of knowing both Rich and Renaun for awhile and met them in person at Adobe MAX 06, and hope to meet Simon at some point. I know from first hand knowledge that these guys are extremely knowledgeable at Flex, and what I would classify as extreme experts.
Though that doesn't necessarily translate into the ability to write well - but fortunately they have the skill, and you can see from their blogs that they're passionate about teaching and sharing knowledge.
This is a good sized book weighing in at 687 pages - and the approach they take is cover a lot of things at once, and progressively ramp up in complexity. The content leans towards the breadth side of things, but they quickly jump straight into deep territory in the sense that they reveal the low level details of how Flex works. With that being said, although a useful book to have as a beginner, they don't spend too much time on "newbie" stuff, and focus more on real world issues you'll encounter. One highlight of the book is the examples; they're very practical and universal, and you get some useful nuggets of code you can use in your real world applications.
I'd recommend this book to anyone, but those that will get the most out of it are intermediate level users who've conquered the rudimentary basics Flex and want to take their game further, and need that next level of understanding. It's also a decent reference book if you want to quickly brush up on a certain topic, or if you're struggling with and issue and need further understanding on the subject to conquer it.