As a small business owner I'd have to say E-Myth Revisited and Work the system. Because once you learn systems thinking, you see the world totally differently

As a small business owner I'd have to say E-Myth Revisited and Work the system. Because once you learn systems thinking, you see the world totally differently and from a business point of view, understanding how their systems work is like getting to have a look at the source code of that business.