Newbie Flex Question

Hi,

I am about to start learning Flex to eventually built some RIA applications and was wondering if everything (preloader, panel, form elements, …) are all customizable as in most sample, I mostly see all the same interface element.

So, is it possible to customize everything in Flex2 ??

Also, is any of you folk know a good Flex2 book. I work since 5 years with Flash & know AS2 and a bit of AS3… so thank to share some book review!

turb