Now, despite the highly deceptive Post title, I myself am no expressions expert.
What i do have is a list of very useful resources that i run to when i need to add more refined or automated animations to my projects. If you frequent CreativeCow and the like you would probably be familiar with a few of the sources that are in my list. Not everything is web based, because despite our digital animatoryness, sometimes learning feels more official when you do it out of a book... just sometimes.
Helpful AE Expressions resources
- http://motionscript.com/index.html - the guy Dan Ebberts himself! He is a master of all things expression and he knows his java, math and AE better than anything else i know. He has a load of mini-sites spawning from this central page so check them out
- https://helpx.adobe.com/after-effects/using/expression-language-reference.html - Well Adobe made the software so only right their very helpful reference page gets on here. So much to learn so little time, i frequently come to this page to check I can do what i think I can.. and also to check my spelling wight he official Adobe list... SPELLING IS VERY IMPORTANT!
- https://forums.creativecow.net/adobeaftereffects_expressions - THE COW! this place is so important to the Creative Communities Of the World (Clue is in the name) The main guy himself (the aforementioned) Dan Ebberts kind of lives on here as far as i can tell, so handy if you need feedback on an expression. Or just want to read otehrpeoples problems and apply them to your own. (thats what i do... never personally had to post).
- http://www.videocopilot.net/basic/ - Kramer is another "Thought Leader" in the Ae world. Watching his vids got me started in this game. With his phenomenal success his webste and product offering has grown and grown. But buried in his site are this great tuts if your starting out fresh in AE and #9 is dedicated Expressions. Check it out if your new to this world!
- After Effects Expressions by Marcus Geduld - Great resource. Of course it is an actual printed book. But if your order it online you still got to use a computer in the process. It lives next to my computer.
So that's the end of my list. You'll probably find a combo of a few of these together will open up a world of animation control to leaves keyframes in the dust (though still very important). They seem scary at first but i find learning them as i need a new level of control is the best way to make them stick. Start with wiggle... :)