Peter Northouse's Leadership: Theory and Practice is a very popular textbook. It covers a large number of leadership theories and discusses practical application of them. It is a bit dry, but it is easier to understand than more complex textbooks such as Yukl's Leadership in Organizations.
If you'd like a less theoretical approach, Kouze and Posner's Leadership Challenge tends to be much appreciated by students.
I think that book is preferable: John Carver (2006) Boards That Make a Difference: A New Design for Leadership in Nonprofit and Public Organizations, 3rd Edition, Jossey-Bass.
This book offers an empowering and fundamental redesign of the board role and emphasizes values, vision, empowerment of both the board and staff, and strategic ability to lead leaders.