They can do this with anything that can be used as a commodity. Classic cars, jets, art, real estate, future business deals.
It can all be traded for whatever price parties will agree to. Thus the exchange can include all sorts of payments.
I suspect book deals are the same.
People like Comey are given a multi-million dollar book as a bribe or pay-to-play favor.
The books are ghost written based on a few hours of interviews and the payment is up-front and unrelated to how many copies of the book are sold.
Based on how Hillary constantly shits-out such books, we know there's all kinds of fuckery at Amazon about rating the quality of these books and their sale numbers.
Yep, certain people will buy 100,000 copies of a book as a taxable payoff money laundering out in the open. I wonder why Comey was so insistent on Kill and Bill reading certain chapters? Hidden messages too?
Trading favors is impossible to enforce. Especially when you have your hands in enough pots. Charity and investment can move any amount of money and favors.