The best single source I'd recommend to get started learning Flash would be: http://www.lynda.com/. However, a larger question is how do you intend to use it, or what is your goal with developing learning materials, and what is your interest/tolerance for programming? Doing work directly in Flash requires more than a little scripting knowledge/skill level. If you're not into programming, there are many solid elearning dev tools that produce excellent Flash output but require little programming knowledge. Articulate, Lectora Inspire, or Camtasia are popular examples. And of course if you're interested in mobile output you want to think about HTML 5 as Flash isn't everywhere in that space. Good luck.