I think the most important book that every future business owner should read is From Impossible to Inevitable by Jason Lemkin and Aaron Ross. The book is focused on SaaS technology companies, but frankly a lot of the lessons contained within are also completely relevant for other types of small businesses.
They focus on customer retention and strategies on how to deepen and improve relationships with your customers. This is important regardless of whether you are a software company or a restaurant. I see Jason Lemkin as one of the greatest business thought leaders of our decade, particularly with respect to small businesses and start-ups. I found this book to be absolutely jam packed with great strategies to handle the thousands of business situations that will come up for the leaders of a small company.