I wrote a reference book on programming and, while the book took a tremendous amount of effort, it was rewarding in many ways.

I became a better communicator, I learned a bunch about the programming language in question, I built a career off of it (with the help of some very kind and wise friends), made my mother proud and so on.

However, it was not that financially rewarding – it was hundred of hours of work for relatively little direct compensation. Of course, the indirect compensation (like being able to get much better consulting gigs more than made up for this.)