There's a huge grift in right wing book media.
They ghost write a book, buy it with campaign/superpac/donor/megachurch funds, and give them away at rallies, etc.
This has 3 benefits: funnels money to the "author", reinforces loyalty of those that get a free book, and makes them appear popular+legitimate by being a "new york times bestseller" (etc) which also gets them in more bookstores as you've described.