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.
I can't really furnish you with a link, though I saw some pretty convincing breakdowns back when I was on Twitter. Long story short, nobody is actually buying e.g. James Ball's books โ they don't even print nearly enough to cover what the author is paid. It's similar to the columnist grift, which is definitely a thing in the US too