Maybe I'm in the wrong part of the Apple dev community, but I don't know of anyone who makes a living on books and speaking. I know many who write books and do a lot of speaking, but there isn't enough money in it for it to be anything more than a side gig.
Yeah, there's certainly some people who would like to make a living on books and speaking, but every single one of them that I know of has a job as an iOS (or macOS) developer of some sort and no one's even in the range where they can pretend that maybe one day they'll be able to quit their day job.
