What leadership book has helped you grow the most in your own leadership?

How to Win Friends and Influence People by Dale Carnegie is one of my go-to's.  As entrepreneurs, we don't spend enough time revisiting the good, bad and ugly of our past work and processes. Too often, we're putting out fires, handling all the things while also looking towards whatever comes next.  In many cases, taking a look back at similar occurrences will help spot trends and those trends can help develop new processes.