O’Reilly continues to publish very good ActionScript books.
Here is the link to their Flash and ActionScript section:
http://www.oreilly.com/pub/topic/flash
Besides the excellent Essencial ActionScript 3.0 book already discussed here, there are 2 more books I reccomend:
ActionScript 3.0 Design Patterns (advanced level)
ActionScript 3.0 Cook Book (more Flex-oriented)