Are there any estimates sales rankings for chess books? How much do a bestselling chess book generally sell?
Obviously, how much money book selling is a business secret. Nobody will tell you, you will never get the information.
But I do work with book publishers, they can comfortably pay my invoices with good profit margin. Furthermore I sell few database apps on the app store (www.smallchess.com), while I'm not a publisher I make non-trivial returns.
Selling chess books won't be make another Jeff Bezos, however, it's still a profitable business. In fact, selling books give better returns than being a 2700+ grandmaster. Unless you're like in the top 20 of the world, selling a good book (or running a business) makes more money.
Why? While the chess marker is much smaller than say football, the chess market is dominated by high-income earners. Chess books buyers have good purchasing power, because if they don't they would have better things to do than chess.
Well known book writers rely on commissions from their books for living.
Amazon publishes best sellers lists by category, and here is the one for chess.
This is obviously not perfect since it's just for Amazon (although they do have what one might call a significant share of the market), and it doesn't tell you anything about how much does a bestselling book generally sells, but I hope it's better than nothing. Also if you look at the page for a specific book, it tells you its ranking among books in general. For example, for Bobby Fischer Teaches Chess it says "#3,480 in Books" as of this writing.