Several of After Effects training guru Harry Frank's tutorial series are now free: After Effects Expressions, Trapcode Particular, and Trapcode Form.
As a thanks for the support, Harry made After Effects Expressions a free download. It may be getting a bit old, but it generally covers the most important aspects of creating expressions in AE. In addition, see the PVC resources CMG Hidden Gems: Chapter 37B and 37C – Expressions and Scripting Bonus Chapters by Chris and Trish Meyer, Deeper Modes of Expression: The Index by Chris and Trish Meyer, and AE Expression Shorts Complete Series by David Torno.
Here's the summary description, and a sample from Harry:
Targeted at the designer, this 3 hour series teaches you how to use expressions in every day projects. This series has quickly become the go-to place to efficiently learn After Effects Expressions! Includes all project files and a Quick Reference PDF to aid in learning. The six lessons are as follows:
- Introduction to Expressions
- Introduction to Arrays and Variables
- 3D Text: Arrays and variables continued
- Random Numbers and Text Strings
- Expression Control, Randomness and Probability
- Audio to Keyframes
The newer Trapcode training series at Red Giant are newer versions of somewhat older training sets. Both Trapcode Particular Getting Started Tutorials (8 videos Trapcode Particular) and Trapcode Form 2 Training (9 videos on Trapcode Form) are nice overviews. Here's some samples: