To some extent, this is true of many things where you do work in advance and then try to sell it. That’s just what a lot of business is. In this case I assume t

To some extent, this is true of many things where you do work in advance and then try to sell it. That’s just what a lot of business is. In this case I assume the author got a lot of feedback over the years in the form of ‘I’m a programmer and I liked your article and I always wanted to learn mathematics and never learned how’. And, like, that sort of thing doesn’t mean writing a book on the topic would be successful, but it does seem like a bit of a hint. I think in general the answers might be something like ‘look for clues’ or ‘try to get feedback on whether the idea is good early’ but I also think a lot of the time it’s just people writing books they want to write or that they guess people will be interested in. But I’m definitely guessing and I don’t really know.