I'm looking at him right now — https://github.com/kovidgoyal. You can sponsor him at https://github.com/sponsors/kovidgoyal.

It seems like he's spending a lot of time developing Calibre and has only 9 sponsors. Do you know of any other way he can make money out of Calibre?

GitHub contributions graph shows that he has consistently worked on Calibre for the past 12 years. That's amazing!