you did not specified whether flex 3 or flex 4...
For Flex 3 - Adobe Flex 3: Training from the Source .
I like this book as it gives you almost all the things that is needed for beginner then you can go for advanced book for flex
For online resource
It is the best way u can learn..
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"best" and "flex" do not go in the same sentence.
i like the cookbook for Flex 4:
it is a good overview, but still doesnt teach me what I need to know to build real and dynamic FLEX apps.
one thing I do not like about this book is the lack of illustrations or screenshots. how are you going to show me 100s of pages of MXML and no photos of what it renders like? typical!
There are lot of books available in the market the best thing i have ever seen is live documentation of the adobe flex3 and u can have another reading help documentations for flex4..
Come to books wise u can go for flex cookbook and programming flex3 and training from the source ,, and also we have some more books that will give some more information i.e learning flex3 and advanced flex programming book ..
these books are enough.. and if u have an command over these books concepts u can develop an extended applications from the flex...