General flash or Actionscript?
Actionscript, you can't get much better than
ActionScript: Actionscript for Flash MX by Colin Moock
General Flash
Pick up one of the Visual Quickstart guides.
or
"Macromedia Flash MX ActionScripting: Advanced Training from the Source" by Derek Franklin, Jobe Makar
Any of the Frends of Ed books are good.
I also like:
ActionScript for Flash MX: The Definitive Guide, 2nd Edition by O'Reilly.
I use flash alot, currently on top of my toilet being read Flash Deconstruction by Juxt Interactive, Php For Flash Friends of Ed, Server Side Flash by sanders/winstanley. All of which are good books for more dynamic flash.