The next one sold none. Which is expected since I made it openly available from my site. It hit about half a million downloads in the first year. Alas, most of them were from one IP somewhere in Czechia so I guess someone's spider got stuck in the web. But real people downloaded it too, about 12 000 copies. I even got a positive review from John D. Cook of which I'm extremely proud https://www.johndcook.com/blog/2020/04/24/programming-langua...
I'm now working on Geometry for Programming book for Manning https://www.manning.com/books/geometry-for-programmers. This would be my first collaboration with a publisher and so far it looks like it will become a moderate success. The early access sales are good, the reviews are encouraging. Even though, I would expect first year sales to be in thousands, maybe even low tens if we're lucky but no more. Definitely not enough to live of.
So while the post doesn't reveal any specific statistics, it does agree with my experience. Most books might indeed sell in very low numbers but they are not really published and maybe not even really books. If you're working with a well established publisher, you will get some moderate sales numbers guaranteed, and a even small chance to go big.