I have been using Flex for 6 to 8 months now. I have used the two books listed. i am particularly found of the book by Gassner.
Flash Builder 4 and Flex 4 Bible [Paperback]
David Gassner
Adobe Flex 3: Training from the Source [Paperback]
Jeff Tapper
Hope that helps!
Eric Andersen
Dept of State Lands/ State of Oregon