I am a voracious reader and regularly read all kinds of books. However if you ask me the name of one book that has had the biggest impact on my life, it surely is Crucial Conversations.
I got some valuable lessons in straight talk and candor from Professor Joe Bentley at University of Utah, and this book builds on those lessons. The framework presented by the authors is great because it works. This book teaches you the skills to conduct the most difficult conversations in all walks of life. It teaches you to confront without spoiling relationships. And it shows you how you can improve your effectiveness by orders of magnitude.
If you could read only one book, I would recommend this one. It has the potential to change your life. It certainly changed mine.