Before i start down with ESRI FLEX API i had to start with Flex itself, having a Adobe Flash 8 Experience, i didn't find it much difficult, but with some new concepts and it was easy to grasp.
I would recommend the following book to anyone starting Flex 3 with Adobe Flex Builder.
OReilly.Learning.Flex.3.Jun.2008 (Read This First)
O'Reilly - Flex 3 Cookbook - May 2008
Oreilly Getting Started With Flex 3 Jun 2008
UPDATE
To get ur self more familiar with the contols and how to apply style to them i recommend the following links
Flex Component Explorer
Flex Style Explorer
