Expressions book by Marcus Geduld
Marcus Geduld just released a new book, After Effects Expressions, on Focal Press.
The fine folks at Focal Press have made a couple of excerpts of this book available on the Web. These samples have some great example expressions, as well as some explanation of how some of the trickier parts of the expression system work.
In the “Physical Simulations” section, Marcus shows how to use expressions for orbits, bounces, jiggles, and collision detection. I know that math is scary, but just learning to use some basic trigonometry can make realistic animations so much easier. Marcus shows how.
(BTW, Dan Ebberts also has an excellent page on collision detection.)
And, of course, there’s a lot of information in the “Expressions” chapter of After Effects Help: