For improving as a manager:
"Understanding and Changing your Management Style," by Robert Benfari
"The No Asshole Rule: Building a Civilized Workplace and Surviving One That Isn't," by Robert I. Sutton
For learning about management styles:
"Primal Leadership," by Goleman, Boyatzis and McKee
"Getting to Yes," By Fisher and Ury
"Difficult Conversations: How to Discuss What Matters Most," by Stone, Patton, Heen and Fisher
For motivating employees:
"Intrinsic Motivation at Work," by Kenneth W. Thomas
For changing your organization:
"Making Sense of Change Management," by Esther Cameron and Mike Green
See Jill's Book Picks for Managers for more.
And weigh in with your favorites. For instance, I think Andy Grove's "Only the Paranoid Survive" is a terrific book about how to manage organizational change.