Because ActionScript and JavaScript are both based on the ECMA-262 scripting language specification, they share a common core syntax. Hence, although the following resources do not focus specifically on ActionScript, they are worthwhile reading for ActionScript programmers:
http://www.ecma.ch/ecma1/STAND/ECMA-262.HTM
The official ECMA-262 language specification from ECMA, the body that maintains the language upon which ActionScript is based. Netscape's JavaScript and Microsoft's Jscript are two other implementations of ECMA-262. This is a highly technical document used to create things such as a conforming ECMA interpreter.
http://developer.netscape.com/docs/manuals/js/core/jsguide/contents.htm
Netscape provides very readable documentation on JavaScript in this detailed explanation of JavaScript's core language features.