If there is only one skill we could ever learn in life, I would suggest it is understanding our own mind.
Beyond that, I would recommend "10% Happier" as it is very pragmatic and belief agnostic. Good for newcomers. Next, I'd recommend "Buddhism Plain And Simple" (Steve Hagen). You do not need to follow Buddhism or have any intention to engage with it as a philosophy to wring a ton of value from the book re: how the mind works.
Anything in the warrior literature will help you find yourself, your limitations and help unlock your potential.
Way of the Peaceful Warrior, by Dan Millmann, is a very good introduction to the whole subject. It's fairly easy to read but absolutely full of little gems of wisdom if you're willing to see them. Once you get that, look for the heavier stuff.