Book recommendation: The Subtle Art of Not Giving a F*ck

There is one question I get posed more often these days: How can I or person xyz learn to better reflect?

What seems like a trivial question is actually rather difficult. Sure, we are familiar with our own thoughts and we all try to make sense of that mess that can inherit our heads. But to reflect in a way to actually derive insight, understanding and an idea how to adjust and change is not that common.