After Effects Expressions by Marcus Geduld is a new book on expressions in AE from Focal Press. Focal describes the basic features:
"Foundation skills set you straight on the essentials and a practical guide to implementation shows you how to 'roll your own'--from the simple to the complex. Full color illustrations display the coding, and pickwhip techniques as well as sequential animations. Downloadable companion files include Quicktime movies of the demo animations, AE project files that permit you to examine the Expressions, and an extensive library of Expressions..."
Amazon lets you peek inside but only a tiny bit. You can see the author in action on at least 2 training videos; check out videos Making key frames loop in After Effects and The"wiggle"expression in After Effects.
Other AE Scripting Resources were surveyed in a previous post, Scripts Galore and as Panels in AE8.