That's a great point. It's not just about I have a book. But that, especially if it's through a publisher (which has pros and cons), I have sufficient knowledge of the topic either acquired day-to-day or through research that I'm capable of laying down at least 250 pages or so of coherent writing on said topic. The last book I did I was definitely already familiar with the area but I was certainly more so my the time I was done.