My experience in writing a technical book 8 years ago was the same: https://www.brentozar.com/archive/2010/01/the-economics-of-w... I made more money from Amazon affiliate fees linking to our book than I did the actual book. Way more. If you’re out to make money short term, you’re better off writing book reviews then books. Having said that, the book gave me some credibility with clients for a couple of years when I…
You would be surprised. Even having the wherewithal to put together the content, put up a site and publish is a huge differentiator. 99% of the people you're competing with in any given niche aren't willing to put in even that much effort.
Beyond these attributes as proxy measures for job-related abilities, having someone like this around can also help attract and retain other quality engineers — so long as the side hustle doesn't displace the work responsibilities (unless the job is re-cast such that those coincide).