The book is just there as a gift within a grift.
The candidate "writes" the book and makes it available for sale. But not even their fans buy it.
The books are ordered at full price despite that by PACs to be given away at stuff like this.
This funnels money from PACs directly into the candidates pocket.
A couple years ago it was a big story, because one of them was strategically doing it thru the specific stores around NYC that were sampled for best seller list. They ended up pretty high up on the list for a couple weeks. Media even interviewed workers at the store who were really confused. Books come in on a schedule a couple skids at a time, all individually packed and not at bulk prices. And not even stay a day until picked up by another truck and shipped off again. At some stores this was more volume than every other book that went through the store normally.
They were burning money like crazy, because it was money laundering. The entire point is to move as much money around as possible.
I wouldn't be surprised if one tried some bullshit with valuating the signature tho.
Have the PAC buy them, pay the candidate to sign them, and then give them out at events like this. That could double maybe even triple the money laundering, but I don't know if they have the balls to do it.