I think the main thing is "a decent book deal". Most authors do not have these. Most academic books are not read outside of academia. Even a book that gets publ

I think the main thing is "a decent book deal". Most authors do not have these. Most academic books are not read outside of academia. Even a book that gets published and does well within academia (gets on many courses reading lists) will not give the author an income. A decent book deal is the very rare book that is both used in the academy and which goes mainstream - something like A Selfish Gene, or a Carl Sagan.