Cyburbia is a friendly big tent, where we share our experiences and thoughts about urban planning practice, the built environment, planning adjacent topics, and anything else that comes to mind. No ads, no spam, and it's free. It's easy to join!
No good ones related directly to planning that I have found. Read Paul Hawken's book about growing a business. Don't start unless you and everyone you care about have explicitly discussed the uncertainty and are committed to it.
I suppose it depends on what kind of business you are looking at starting. Are you going into consulting? The APA bookstore has a couple of books on that topic. Are you planning to be a developer? I would recommend several ULI publications - their "handbook" series is very good. I am starting a small manufacturing business. I suppose it helps to have a background in economic development. I have found pretty much all I need online.