2) Having a general understanding of basic business structure. Knowing the basic difference between an LLC, C-Corp, S-Corp, and sole proprietor is good. You don't have to know all the differences or legal intricacies, but having a general idea of the difference is good.
3) Depending on your hiring plans, some knowledge basic employment law can be helpful.
4) Basic tax laws and filing requirements for your state. Do you need a business or professional license? Are their quarterly, annual filings required? When are they due?
Not sure of a book, most you will learn over time, but a general intro to business book may be helpful. Because laws can vary by state and even county or city, it will be tough to find one source for everything. That is why #1 above is important in my book. Unless you are an attorney it is almost impossible to know all the laws. You can easily fall victim to the "know enough to be dangerous" theory when it comes to law or accounting. It's good to understand what they are talking about at a high level, but let the professionals handle the details.