I think you'd enjoy ActionScript for Flash MX: The Definitive Guide by Colin Moock, it is by far the best and most comprehensive book on actionscript i've seen. It also has a massive function reference that shows the syntax and potential bugs along with practical examples for just about every function/object in actionscript.

To detect key presses you might create an object that "listens" for events triggered by the Key object, this object can then respond accordingly,

Code:
var kl = {};
// create a new object {} is a shorthand for new Object();
// define a method to be triggered when an onKeyDown event happens
kl.onKeyDown = function() {
	switch (Key.getCode()) { // what happens depends on which key was pressed
		case 32: // 32 = the space key was pressed
			trace("space");
		break;
		case 49:
			trace(1);
		break;
		case 50:
			trace(2);
		break;
	}
};
// make the kl object a listener of the key object, 
// (so each time the key object broadcasts a message saying 
// a keyDown event happened the kl object will know about it
Key.addListener(kl);