Math insight: many clear explanations about operators we usually don't understand...
MIT on-line courses (MITOPENCOURSEWARE - MASSACHUSSETS INSTITUTE OF TECHNOLOGY), I took the following ones:
Finite Element Procedures for Solids and Structures (by Klaus-Jürgen Bathe !)
Differential Equations in Action
Programming Foundations with Python
Intro to Physics
Excerpts from Stephen P. Timoshenko's auto-biography...