Quiet Leadership by David Rock. Improving your performance at work by changing the way you think. Rock uses a brain-based approach that is easy to read and meaningful for anyone looking to improve their own performance as well as their team's.
The Leadership Challenge by Jim Kouzes and Barry Posner. Classic book with practical and inspiration lessons in leadership based on research from over 60,000 leaders. Solid, concrete concepts and behaviors that successful leaders utilize - and which are learnable for those moving up the leadership ladder.