I agree with the general sentiment about Dane and Tim's answer that learning business law in depth can be a waste of time, but only if you already know a good deal about the basics.
I think if you are contemplating starting a business you need to have at least a minimal knowledge of the law. In startups you need to know about NDAs, contract/tort law, employment laws, corporations/partnerships, intellectual property law (patents, trademarks, copyrights, trade secrets), vesting contracts, and so on.
Not knowing the basics about these will certainly get you in trouble. I disagree with Dane's statement that you should only learn about this when you need to solve a problem. That is the costliest time possible to learn that you broke a law or that your contract is not valid, or that you don't own or control the intellectual property you worked hard to create.
You should become conversant about day to day topics before you do anything that might bring you big problems in the future. Some questions on this board have been very indicative of the problems you can run into if you are not proactive: The guy that wanted to know how to force his employee to work 60 hours a week; the other person that wanted to know how to use trade secrets he knew about his current employer to start a competing company (plus poach customers, if I remember correctly); the guy confused as to whether or not he had to release the source code of his application if he extended an existing open source product; the list goes on. There have been many really good questions about law on here.
These issues have a huge impact on a startup, especially if they are ignored until they blow up. And they are everyday issues. So I think you are wise to be proactive and learn about business law.
Having said all of that, you asked for a book title, so there is a book I like, called "The Entrepreneur's Guide to Business Law." It's got good stuff on there. There might be other good book out there, maybe other people can suggest more.