Summary/High Level Thoughts
My (relatively) low rating isn't necessarily reflective of the principles in the book; those are obviously timeless and things everyone should know and practice. That, rather, is the reason for the relatively low rating.
I'd still recommend everyone read the book once - it's a quick read - but the principles inside won't likely modify your thinking to a great degree, or cause a large shift in your perspective.
Rather, they're things that should be obvious, and can be summarized fairly effectively as they are below.
Don’t criticize, condemn, or complain.
Give honest and sincere appreciation.
Arouse in the other person an eager want.
Making People Like You:
Become genuinely interested in other people.
Remember that a person’s name is to that person the sweetest and most important sound in any language.
Be a good listener. Encourage others to talk about themselves.
Talk in terms of the other person’s interests.
Make the other person feel important and do it sincerely.
How to Win People to Your Way of Thinking:
The only way to get the best of an argument is to avoid it.
Show respect for the other person’s opinions. Never say “you’re wrong”.
If you are wrong, admit it quickly and emphatically.
Begin in a friendly way.
Get the other person saying “yes, yes” immediately.
Let the other person do a great deal of the talking.
Let the other person feel that the idea is his or hers.
Try honestly to see things from the other person’s point of view.
Be sympathetic with the other person’s ideas and desires.
Appeal to the nobler motives.
Dramatize your ideas.
Throw down a challenge.
How to Change People Without Giving Offense or Arousing Resentment:
Begin with praise and honest appreciation.
Call attention to people’s mistakes indirectly.
Talk about your own mistakes before criticizing the other person.
Ask questions instead of giving direct orders.
Let the other person save face.
Praise the slightest improvement and praise every improvement. Be “hearty in your approbation and lavish in your praise.”
Give the other person a fine reputation to live up to.
Use encouragement - make the fault seem easy to correct.
Make the other person happy about doing the thing you suggest.