the best way - read docs.
http://livedocs.adobe.com/flex/3/devguide_flex3.pdf - developer guide.
http://livedocs.adobe.com/flex/3/langref/index.html - lang specification
list of all available docs: http://www.adobe.com/support/documentation/en/flex/
Don't waste your money and time on a book. Start by reading these LiveDoc pages:
I read 3/5 of the Flex Builder help sys and got ACE certified. It may not sound attractive, but it is THE best way to learn Flex, in addition to practicing actually creating some apps.
If this post answers your question of helps, please mark it as such.