If you want to get started with Flex there is no better way than sitting yourself down and start experimenting with this book next to you on the desk. Although there's not much choice of books on Flex, this one I've found is exceptionally well-written and will guide you through all aspects of RIA development with Flex. Something to take into consideration is that "Developing Rich Clients for Macromedia Flex" is not a book for absolute beginners - you ideally need to have some background knowledge on ActionScript 2.0 and OOP methodologies before you get started.

The guys from iteration::two (soon to be Macromedia Consulting EMEA) did an excellent job on this book, hope they'll do an update for the next release of Flex.