I read the book The Four Agreements in one afternoon and I don't remember ever reading any other book in one afternoon.
The Four Agreements are:
1. Be impeccable with your word
2. Don't take anything personally
3. Don't make assumptions
4. Always do your best.

the gist of his message is:

All of us grow up with teachings and expectations that condemn us. What's worst we keep on condemning ourselves about our flaws and weaknesses. Know the power of our words and say only wholesome and helpful words. Do not accept condemnation from others personally. It is their points of view arising from their own perceptions. Do not make inaccurate assumptions that lead to unhappiness and misery. Ask and clarify if you have doubts.Move on if we must. Our best varies with age, state of health and circumstances but whatever the condition just do our best. Heaven or Hell is within us. Chose Heaven.

To make easy to remember I have simplified it to the 4 Bs:
1st B: Be a speaker of integrity, not the gritty-nitty.
2nd B: Be immuned to the words and actions of others, not be infected by the virus of what others do or say.
3rd B:Be realistic, not make false or imaginary assumptions.
4th B: Be the best that you can be, not the best of others and their expectations.