How many books do authors sell each year?

Suppose you make a 500 realistic checkmates problem book, and you are expert strength but check them with computers. You try to sell them online for $15 each. How many do you think you'd sell in a year, if the book description, order, and problem types were in demand and the competition missed some of your important factors? 1000 books in a year? After shipping, printing, and driving to the post office, if you make $5 per book as profit, that is only $5000 per year, until a competitor copies your problems or follows your same revolutionary format and your profits then drop to maybe $2000 per year. With the yearly profit margin pretty close to up front bulk printing costs, it almost does not seem worth it.

Does that sound right? 1000 books per year? How many chess players are actually in the market to buy books, and how saturated are they already with book? How many just look around online for free stuff? I think a chess author would do well to make books accessible to Russians and Cubans in order to sell more, but maybe those would prefer to buy from GMs in their own language.

If they are smart enough to make a book about chess they are probably good enough to play touraments, so the money they make on the book is just extra. Then they might even have a job outside chess. That is just my opinion.

I agree-- probably in almost any profession, for the vast majority of people the income they get from writing is just one component of their total income.  Writing adds extra cash and gets them some publicity that might make it easier to sell future articles and books, but I doubt it's the main source of income for many people.