Which book or course gave you an unfair advantage?

A lot of them have. Which field are we talking about? It would take me a while to list all the books and courses.

The best are religious (even the ones you don't follow, nobody ever RTFMs). But the best agnostic one I'd pick now would be High Growth Handbook; lots of advice from people who have made very expensive mistakes from people who play at the top of the game for a few decades. Lots of the advice is not always true, but having such a toolkit is invaluable. Otherwise we make the same mistakes others have made 20 years ago on the other side of the world.

I dare say the difference in skill is not from experience, but from learning from other people's experiences. And the best books need to be read multiple times. Sometimes with another person who can grok the real meaning.

No book or course has ever done that for me. The best books give me a deeper understanding of a topic, which is hugely valuable, but none have been game-changers in the way you mean.

Being healthy and fit. Having a strong immune system. Gut health. Eating healthy and well. Getting good sleep. Proactively taking care of your body and mental health.

CBT Therapy is very good. Teaching one that, one's perspective is everything.

Being social, having positive friends.

If there was one book, that changed my mindset from a negative thinker to a positive thinker. Think and Grow Rich, by Napoleon Hill.

And the secret sauce to high performance. Adderall. Alpha-GPC is also very good and worthwhile to try first.