My clear choice would be How to Win Friends and Influence People by Dale Carnegie (1888-1955). It is probably the most important book on management written in the past 200 years.
It has been more than 75 years since the first edition of this book was published. Some of the references might be outdated for some of us, or not entirely familiar to readers from outside North America. However, How to Win Friends and Influence People remains a timeless guide to leadership and management in any business setting, and is especially relevant to law firms and corporate legal departments. It is also an unpretentious collection of simple wisdom.
Norman Clark