The Leadership Challenge, 4th Edition by James M. Kouzes and Barry Z. Posner. This is a tremendous resource we have used for years. It was a favorite from my MBA program's Organization Behavior class.
Becoming a Person of Influence: How to Positively Impact the Lives of Others by John C. Maxwell and Jim Dornan. I have used this book in mentoring co-workers over the last two years. Lots of topics for good discussion of real life issues.
StrengthsFinder 2.0: A New and Upgraded Edition of the Online Test from Gallup's Now, Discover Your Strengths by Tom Rath. All my team members have been coerced to take this test. We have shared the results and find working together easier and more efficient when we know our strengths and the strengths of others.
The Secret by Ken Blanchard and Mark Miller. This is a book that really gets to the heart of what makes a leader truly able to inspire and motivate people.
Fierce Conversations by Susan Scott. A great explanation of how to transform everyday conversations with effective ways to get your message across... and get what you want.
New Horizons for Afghanistan by Arley Loewen. A great book teaching how to be better leaders of influence in any position of responsibility.
Leadership and Self-Deception by The Arbinger Institute. Discover what self-deception is, how we get trapped in it, how it undermines personal achievement and organizational performance, and most importantly, the way to solve it.
Getting Naked by Patrick Lencioni. A business fable about shedding the three fears that sabotage clients' loyalty.
The Five Dysfunctions of a Team by Patrick Lencioni. A fantastic analysis of what makes teams work effectively.