But is most self-help, as measured by sales? I haven't looked at sales figures, but I imagine it's not that unlikely that half of all self-help books actually consumed are the ones that are genuinely helpful.

"genuinely helpful" is a complicated term. A lot of books bring people to shift their attention to different priorities and get better at one thing while sacrificing other things.

New Agey literature about being in the moment has advantages but it can also hold people back from more long-term thinking.