The greatest references around :
1/Dan Ebberts' : http://www.motionscript.com/
A very good introduction with a lot of sample expressions, clearly explained and commented
2/JJ Gifford's : http://www.jjgifford.com/expressions/
A slightly more advanced intro, and also a lot of examples
Good javascript reference and guides :
1/Javascript central, where you can download Javascript references and manuals (free) : http://devedge.netscape.com/central/javascript/