Blogs with valuable insight about leadership:

Leadership Freak

Strategy + Business

Leadership Development

Daniel Goleman on emotional intelligence and leadership

The Art & Science of Leadership

Books I recommend:

The Leadership Engine (Collins Business Essentials)

Self Leadership and the One Minute Manager: Increasing Effectiveness Through Situational Self Leadership

The One Minute Manager Meets the Monkey

The New One Minute Manager

The 21 Irrefutable Laws of Leadership

Leadership and the One Minute Manager

The One Minute Manager Builds High-Performing Teams

The Five Levels of Leadership

Leadership: Management Skills, Social Skills, Communication Skills

The Seven Habits of Highly Effective People

Learning Leadership