Every one of these “future candidates” feel they need to write a book to impress folks with how brilliant they are. Someone else writes the book, and then they go on tours to greet fans and sell most of the books to the DNC.
The most shameless example of the ghostwritten book helping a political career is Profiles in Courage from JFK. Everyone knew in DC it was his aide Ted Sorensen who wrote the damn book, but with some lobbying and arm twisting from Joe Kennedy, it became a bestseller and won a Pulitzer Prize. It gave JFK some intellectual “gravitas” and helped launch him to the presidency.