I'd also recommend Eric Ries "The Lean Startup". Maybe it might be obvious to you - but in one stage of my life, my belief of entrepreneurship involved "writing a detailed business plan" and doing a lot of "top-down" planning that I recognise now, in retrospect, was a time-waster.

I'd also like to recommend looking up Alexander Osterwalder's Business Model Canvas as an answer to the question, "OK, if you're telling me not to spend too much time writing a business plan, what should I do instead?"

I wish I'd known about both at the beginning, not towards the end, of my entrepreneurial journey.