Flash challenge levels are relatively new and currently disappointingly simple. They normally go something like - 1/ Save swf file to disk. 2/ Open swf file in decompiler (I use Sothinks SWF Decompiler). 3/ Read answer and use it to complete the level. When it is decompiled it will consist of various resources and it is the action scripts that you will be interested in. The main challenge of swfs at present are:
Perhaps one area to look out for in the future is director files for which there are currently no decompilers available - who cares to take up the challenge ? (I also look forward to .NET IL crackmes ;)).
The following book is recommended if you want to learn a lot about Flash Actionscript, like writing your own scripts, but purely for solving challenges no book is really necessary up to now as most Actionscripts are extremely simple and you can solve them without knowing the language details.