I have (shamefully) just begun teaching myself ActionScript 3.0 in my downtime. It won’t be as hard as the first time through (learning Flash on my own) since now I understand the basics around code, but there are a lot of new concepts to AS3 that I need to wrap my head around before I start the nitty gritty part of memorizing the syntax for writing AS3.
Not that I am an expert on all AS3 books out there, but I happened to get myself into Learning ActionScript 3.0: A Beginner’s Guide. I think I like it because the first few chapters are basically a refresher on what variables, function, etc. are, and what they actually mean to the flash compiler. They get into detail about the way the compiler actually reads the code, but then it also explains things by comparing it to the real world (i.e. a child has the same attributes that their parents do). That tends to be the best way I learn.
I just found this link to all the source files that go along with the book. Extremely useful.