What Counts as a Bestseller?

And "best selling" as a substitute for quality doesn't hold that well, either.

It's great if you're satisfied selling what you're selling, but placement on the NYT list is more than just an award, it's a huge marketing opportunity. Getting on the list could easily 5x your sales and revenue, and have publishers and booksellers knocking on your door instead of other way around.

Interesting, how do get them manufactured? I guess there are printing companies that operate separately from publishers?

I believe the business model is "sell a few thousand expensive hardbacks to university libraries to cover publishing costs". Authors tend not to make any money (unless they book happens to be particularly successful, which happens rarely), and are incentivised by the desire to spread knowledge. Many are academics who are paid by their employers to write books amongst other activities.

As a decidedly non-professional self-published writer, at present my two novels so far are around the 1M ranking on amazon.com and ~270k on amazon.co.uk... I'm above the single-digit copies per month for them, but not a whole lot, so can vouch that your numbers are pretty reasonable.

But with respect to being surprised, recent numbers from the UK - I wish I remembered the source, suggests the average income for authors from their writing is around ~13k/year, while average household income for writers was ~80k/year.

In other words, even most "professional" traditionally published authors are either doing it as a second job or has a partner in a high paid job. Writing is very much a "job" that for most people is a passion that may or may not even break even, and where only a tiny proportion of writers ever get to the point of making a living.

Part of that is building up back catalog, and building up an audience. I recall Charles Stross, to take someone who is living of it now, mentioned on Twitter a while back that it took him about a decade before he made 5k in a single year from his writing.

To take a more extreme example of this, Georges Simenon (Maigret etc.) at one point sold only ca. 8k copies per new novel from his French publisher. He was still wealthy, but because of the sheer number of novels (he wrote several hundred), number of translations and number of adaptations, but each new novel added just a trickle and mostly served to keep interest up.

A Norwegian author, Kjell Hallbing, mostly writing as Louis Masterson, sold 20 million books before he died, yet all but his last 6 or so sold in tiny numbers[1] But he wrote well over a hundred novels, and they were translated to more than 20 languages before he died (but not to English; the first English translation came years after he died, and they're not great).

[1] The last six represented a return of his most popular series after a multi-year hiatus - his publisher was mocked when they printed a first edition of 100k for the Norwegian market, but it sold out in no-time; ~30 years of accumulating an audience and building nostalgia among people who mostly will have read just a small portion of his books has an effect...