Dan recommends these books... 

Leading Change - John P. Kotter

The Servant Leader Within: A Transformative Path - Robert K. Greenleaf

Leadership - James M. Burns

Leadership and Performance Beyond Expectation - Bernard M. Bass

Good to Great: Why Some Companies Make the Leap and Others Don't - Jim C. Collins

Leadership for the Twenty-First Century - Joseph C. Rost

Getting to Yes: Negotiating Agreement Without Giving In - Roger Fisher, William L. Ury & Bruce Patton

The Leadership Challenge: How to Make Extraordinary Things Happen in Organizations - James M. Kouzes & Barry Z. Posner

Leadership without Easy Answers - Ronald A. Heifetz