I normally don’t pimp out books but I’d like to mention these since I’ve read them recently. The first is Flash Builder 4 and Flex 4 Bible by David Gassner. I was migrating an app from Flex 3 to Flex 4 and ran into numerous hurdles. About 90% of our design had to be reskinned. I found this chapter in his book on skinning, http://media.wiley.com/product_data/excerpt/56/04704889/0470488956-4.pdf from this page, http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470488956.html. It contained details that the official documentation lacked. To be fair Flex 4 came out around March so I think in time, it too will have the same details.
The second book is Flex 4 Fun by Chet Haase. This book covers the graphical and animation side of Flex 4. There are a lot of books that cover the basics. This contains the kind of details an advanced developer is looking for. Take a look at a sample chapter, http://www.artima.com/samples/flex4funCh2.pdf.
I’d like to see more books by these guys so support them!
Disclaimer: I don’t receive any money or free things for this post or any post. Though I should… probably… because I would give you an accurate review either way…but i wont… but could… but won’t….