Below are a number of resources to which you may find useful.
Effective Leadership by John Adair
A brilliant summary of the essentials of leadership.
The 7 Habits of Highly Effective People by Stephen R. Covey.
A great introduction to the study of excellence and a practical application of Appreciative Inquiry in a business and organisational context.
These are available in PDF format, so you will require either Adobe’s PDF viewer, or an alternative.