Definitely start out by doing some thorough research on your market to make sure you'll have a sizable audience. You can also build a landing page for your book *before* you write it, to gauge interest and price sensitivity. Include an image of the book cover, a description of its content, and a Buy Now button, which when clicked shows a message saying "Thanks for your interest! XYZ book isn't available for purchase yet, but enter your email here and we'll let you know as soon as it's ready."

That way you're able to get an initial sense of conversion rate from page viewers to purchases, and you can also A/B test the page with different price points to see how much people are willing to pay for your content.

If there's enough interest, go ahead and start writing! If not, you just saved yourself a good deal of time, and perhaps you can try another topic.