If you're going to speculate like that, you need to provide specific examples of suspect posts and users.

As others have said, it seems unlikely someone is paying to promote a book that's more than a year old. And if they were, I think we'd probably see more action than "biweekly posts."

If you can show that the users making these posts have strange behavior patterns indicating they're bots or paid promoters: Interesting.

Otherwise: meh.