Trump has a new book. Cruz has a recent book. They all seem to have books. That seems to be the fallback plan if they lose.
I seriously doubt Trump is doing it for the money he makes on the book.
I don’t think Trump or Cruz are in it to sell their books. Trump’s books sell anyway and Cruz needed to introduce himself to the electorate nationally.
Carson has already published books but running will definitely increase his sales.
I don’t know if that is the reason Carson is running but I have wondered about it.
The other reason is that he, Carson, doesn’t seem to be doing a lot toward a campaign structure.
He is spending round 80% of his donations on raising more money rather than spending that money to get on ballets and hire campaign infrastructure.