ActionScript for Flash MX: Def. Guide
The title says it all. If you are looking for a comprehensive book
that covers the ins and outs of ActionScript, this book is it!
ASDG2: Pocket Reference
Same as the above, but
more compact and cheaper. Definitely worth considering.
Flash Math Creativity
The ultimate book on combining Math with Flash to create stellar
animations and flash applications.
Programming Macromedia Flash MX
A nice book that walks you through the mind of Robert Penner and his
approach and methods of creating cool effects.