This is a great beginners introduction. It takes a very short time to go through, even while doing the examples and afterwards, you can feel the productivity increase in the command line (if you where a beginner)

Am looking forward to his GIT book

A "for dummies" style take on the command-line. Not a lot here.

When you learn enough X to be dangerous, the take away is you didn't learn X at all. That's why you are dangerous wielding X.

That seems unduly dismissive. There's nothing wrong with introductions, and the title is just an idiom.