The best lessons are ones you actually use and find useful. Instead of reading 500 business books and searching endlessly for the next great idea, you will be better off sticking with a few principles you actually use.
For example, you would do well to live by any of the following principles:
If you help others, you will have a more rewarding life.
if you are honest about your weaknesses as well as your strengths, people will be eager to work with you. (Many leaders fail to understand this, and thus they fail to lead effectively.)
You cannot reach your full potential without collaborating with others.
Don't take yourself too seriously, but always take others seriously.
Never compare yourself to others; compare yourself to your own potential.
Be both confident as well as competent; either one alone is not enough.
You have a far greater ability to change your circumstances than you think. But change requires persistence; you have to keep certain principles in the front of your mind, week after month after year.
When someone asks you about your guiding principles, you should have an answer... and your answer should be the same this month as it was last month.
This is the sentence I use to guide both my personal and professional life:
Be generous and expert, trustworthy and clear, open-minded and adaptable, persistent and present.
You are welcome to borrow my principles, but you are also capable of crafting your own.