I don't think it's astroturfing. It's a genuinely good book, and the people who didn't read it at launch probably discovered it through a friend or randomly, and want to gush over it. My wife read it after me nagging her for a year, and then she would not shut up about it. She's not a reddit-person, but she definitely posted about it on Instagram and Facebook.

Another book with the same pattern is Piranesi - that's not as "available", but it's just as deserved.