I just finished rereading Improving Organizational Effectiveness Through Transformational Leadership (edited by B.M. Bass and B. J. Avolio, 1994). I first encountered this work back around 1999, as part of my continual research on the topic of leadership. Bass is perhaps the most prolific empirical researcher of the construct of leadership, to date. A quick Google or Wikipedia search will provide you a plethora of works he has authored.
In my view his most powerful contribution to the study of leadership is his Full Range of Leadership Model, which is a conceptual "lens" through which we can view the concept of leadership. Below is a quick summary, with the effectiveness range running highest at the top of the list, sequentially diminishing as you work your way down:
Transformational Leadership - the leader impacts followers through four operational leadership skills - 1) idealized influence, 2) inspirational motivation, 3) intellectual stimulation, and 4) individualized consideration.
Transactional Contingent Reward - followers compliantly do what the leader wants, and get rewarded for doing so, somehow.
Transactional Management-by-Exception (Active) - the leader actively monitors for deviances from expectations in follower behavior/performance, then reacts punitively when violations are identified.
Transactional Management-by-Exception (Passive) - the leader does not actively monitor for deviances from expectations in follower behavior/performance, but when those come to his/her attention, punitive responses follow.
Laissez-faire - the complete avoidance or absence of leadership.
We can probably all think of concrete examples in our experience that align with each description. The more important response to me is examine carefully my own behaviors, thinking, and skills, and work actively to move my daily performance toward that Transformational level.