I can't speak to the people writing books on coding, etc., but I have written a lot of product documentation and am thinking about doing it full time as an independent consultant. The small startups where I have worked have had woefully bad documentation. Developers and support reps never have time or want to write it. I would say the most useful thing I have done for these companies has been to set them up with a ba…
Doing this properly is often not the most fun thing that people can be doing, but is extremely important for getting others using your, well, whatever. I remember working with an older colleague who was excellent at building up good documentation and we'd follow whatever instructions were there, hit problems and solve them, add something to the docs then go back to the beginning and go through everything again. In doing so, we could often create scripts to automate sections and improve the setup process itself.
It's a task that can be hard to get devs to spend their time on, requires skill in a few different areas and has some clear benefits (reduce dev setup time / onboarding costs, improve customer support, etc). There's no lock-in and progress and completion can be pretty clear.
There might be a good source of bounties for the same kind of thing for open source projects too.