How to Win Friends and Influence People - This was one of the first business books I read and have kept many of its principles working for me throughout my life. The book really does a great job of helping you understand how to get people to want to help you in whatever it is you're doing. I've found that this book has helped me in countless situations to better understand people and how to interact with people in a way that helps everyone. No matter what your startup, you're going to be interacting with lots of people and will need to influence others regularly.
The Effective Executive - A great book that I think did itself a disservice with its name; it's not just for executives but for anybody that has a drive and desire to be successful. The key takeaway that I got from the book is that time is the most valuable resource you have, and it must be managed religiously. Once you waste time, it's gone forever and you can't get it back. After reading the book, I'm much more protective of my time and what I do with it. I'm better about prioritizing the things I need to do and focusing to get work accomplished. I still like to have some down time every now and then, but find myself more motivated to work so I don't lose that time forever.
The Innovator's Dilemma - This book is another that really changed my view of products and innovation. The author distinguishes between disruptive and sustaining innovation and reviews a number of case studies where companies solely focused on sustaining innovation eventually fail. The author goes looks at the hard drive industry throughout the book, showing the progression of inventions related to different computer types starting with mainframes, and how the hard drive makers failed at each new computer invention to make the changes necessary to survive disruptive innovation. He also discusses how most companies come into an industry as a disruptor but eventually settle into a sustaining mindset that eventually leads to their irrelevance.
The Lean Startup - Introduces Lean concepts to building a company. Lean was started in the manufacturing space, focused on reducing the waste involved in processes. This book connects the idea of removing waste to starting a company. The key takeaway for me was that startups are successful when they master the "Build-Measure-Learn" cycle where the minimum viable product is built, feedback is gathered from customers and data, and learnings are made that validate the feature built or reframe the startup's thinking.