> Don't you have to be fairly well off to invest the time to write a book?
No. I'm aware of plenty of critically acclaimed writers who are not particularly rich. (Although there are plenty of people who got rich because of the books).
I guess most of them have/had other jobs, like teaching at a university for example. And some — especially people who were university teachers — tend to continue on that job.
> Not to mention a probably strong correlation in higher education in an area that is not all that useful for getting a job
A college degree in things like literature, political science, economics, and general science subjects (physics, chemistry, math, biology etc) tend to be inexpensive here, as compared to a degree in engineering or medical science.