How To Win Friends & Influence People - Dale Carnegie
Don't let the weird title throw you off. Although I was forced to read this when I was a teenager, I realized how important and helpful the principles taught in this book is to your career and overall life. This is not a boring read: the stories and lessons taught in this book had me going "ooh I see!" every time.
You will thank yourself after you have picked up this book and will realize how much better you can improve on your existing as well as future relationship.