Leadership writer John Maxwell claims that leadership is influence.  His point is that no matter how smart you are, how wise, how good your instincts or advice, they mean nothing without influence.  If no one pays attention to what you are saying you are not leading.  Influence is necessary to leadership definitionally.

Max DePree writes in Leadership Is An Art:  "The first responsibility of a leader is to define reality.  The last is to say thank you.  In between the two, the leader must become a servant and a debtor."  Similarly, DePree notes that leaders build assets and a legacy; they build and maintain momentum; champion effectiveness and defend civility and values.

Maxwell provides 21 Irrefutable Laws of Leadership to support the necessary influence factor in leadership.

I've found these two texts to be nearly comprehensive on the topic of leadership.  Have you found other good sources?