I stumbled on this link from some substack:
Writing books is not really a good idea – by Elle Griffin
According to Bookstat, which looks at the book publishing market as a whole, there were 2.6 million books sold online in 2020 and only 268 of them sold more than 100,000 copies—that’s only 0.01 percent of books. By far, the more likely thing is to sell between 0 and 1,000 copies—and that was 96 percent of books last year.

Something more:
Many authors hope that securing a contract with a Big Four publishing house will provide more marketing and sell more copies of the book, but the reality is that traditional publishers are looking for a sure thing. They want an author who already has an existing platform and can guarantee an audience. And if the author has that, they might be better off going it alone.
I know of some established authors (and some budding authors) who have to mark up their presence by several notches. While the avenues for marketing have increased (podcasts, YouTube, social media), the breakthrough from clutter is difficult, if not impossible. Book signing events might turn up a few bored, cynical or nosy people but book reading has diminished, if not been eliminated. Therefore, it means catering to a few.
Why am I mentioning this? Our established textbooks on the scene represent the “snapshot” of recommendations. Instead of doing a deep dive on topical issues, they tend to encompass everything. Perhaps, it is the time for niche domain specialisation in radiation oncology, but even then, the collation of knowledge base of ideas requires constant updating and summarisation.
I have some ideas.
It doesn’t involve traditional publication and signing up your rights. It’s democratisation of access. I hope to work on it.