A very good book even it writing 80+ years ago, the message still applicable in today’s world. This is a solid book that helped me so much on my journey to being a better version of myself just by applying these concepts in real social situations. This self-help book was written by Dale Carnegie, published in 1936 during the Great Depression and equally valuable during booming economies and difficult times. Divided into four main parts.

Part One Fundamental Technique in Handling People: Talk to people about themselves. Put yourself in their perspective. Don't criticize, condemn or complain. See it through their eyes. Show appreciation. See what it is that they want and try to align it with what you want. This is the principle of arousing an eager want in the other person.

Part Two Six Way to Make People Like You: Become genuinely interested in other people. Ask them about their profession, what they've accomplished, where they came from, what they aspire to, what they've endured. Understand that people often live as though we are the star our own life movie and it is the only movie.

Part Three How to Win People to Your Way of Thinking: If you made a mistake, don't defend yourself. Own the mistake. Admit that you were wrong and do it emphatically. State how terrible the mistake was, how you could be punished or what repercussions there could be. Do so so that you deflate the other person's anger. This puts them in a position that they'll actually may defend you.

Part Four How to Change People Without Giving Offence or Arousing Resentment: Talk about your own mistakes first.
On criticism
Criticism is futile because it puts a person on the defensive and usually makes him strive to justify himself. Criticism is dangerous, because it wounds a person’s precious pride, hurts his sense of importance, and arouses resentment. …. Any fool can criticize, condemn and complain—and most fools do. But it takes character and self-control to be understanding and forgiving. That reminds me of this famous quote by Thomas Carlyle: “A great man shows his greatness by the way he treats little men.”
On dealing with people
When dealing with people, let us remember we are not dealing with creatures of logic. We are dealing with creatures of emotion, creatures bristling with prejudices and motivated by pride and vanity.
On influence
The only way on earth to influence other people is to talk about what they want and show them how to get it.
On the secret of success
If there is any one secret of success, it lies in the ability to get the other person’s point of view and see things from that person’s angle as well as from your own.

In a nutshell: Be a Leader.
Principle 1.- Begin with praise and honest appreciation
Principle 2. - Call attention to people's mistake indirectly
Principle 3. - Talk about your own mistake before criticizing the other person
Principle 4. - Ask a question instead of giving direct orders
Principle 5. - Let the other person save face
Principle 6. - Praise the slightest improvement and praise every movement. Be "hearty in your approbation and lavish in your praise."
Principle 7. - Give the other person a fine reputation to live up to.
Principle 8. - Use encouragement. Make the fault seem easy to correct.
Principle 9. - Make the other person happy about doing the things you suggest. 

To summarize, always validate the other person and genuinely care for them.