Writing (from a quick skim check) at least 16 books over a period of 6 years, and earning money from that, is not exactly what I'd call "passive income". And given the topics are mostly (all?) about niches in current technologies they also won't continue to deliver passively forever. A more appropriate title and angle might be "How I've earned $10k/book and manage to keep writing more while maintaining a day job". (W…
Some quick napkin-math I found 17 books where he was author and co-author, all books sum up to 2924 pages. I'm sure some of these books have lots of overlap - for example, I found three books which included "React hooks" in them, so I'd assume that he copy/pasted some pages. But for simplicity and worst-case scenario, let's assume 2924 unique pages, and subtract say, 200 blank pages and similar, so 2724 unique pages.…
It likely ends up being a lot higher revenue depending on the author's life style.
Books and courses potentially have a lot of "back-end" value from opportunities that come up through folks finding you through your book / course.
Here's a couple of excerpts from writing blog posts and making a few video courses over the last 5-6 years:
Wrote a blog post about Flask that took me 3 hours to write but it resulted in 40 hours of contract work from someone finding me through that post. You could say the post directly made $0, but indirectly made a lot more than $0. Stuff like this might happen 8 months after you write the post.
In another case, I made a course about Flask and someone reached me out to me for contract work which resulted in a multi-year long gig which then turned into a full time position and along the way met some great people.
I've flown across the US a few times for tech events (all expenses paid) because of a blog post and got to experience things I wouldn't have gotten to experience on my own.
There's dozens of other events and contract gigs over the years that happened from writing blog posts and video courses. They all resulted in either indirect monetary gains or fun life experiences. A lot of the posts resulted in nothing too but that's ok. To me writing is the reward, everything else is secondary.