[ghaff]

I think that's correct about technical books for the most part. Certainly the case with me. Writing the books has been very valuable career-wise. But the money is trivial.

The difference with open source projects is that there are a lot of project leaders who in my experience get sucked into an ongoing full-time role for which they receive very little money. Such a role can lead to a good job of course. But my sense is that there are more people doing a full-timeish largely unpaid gig writing software/managing a project than in the case of technical books--which people are more likely to do as a one-off project that they may even do partly on work time.