Ann Coulter is or isn't selling. One of the things that is mentioned early on but isn't really fleshed out is the issue of how much visibility Coulter gets. Politics aside, usually the best way to sell a book these days is to be controversial somehow, or at least controversial to some. I don't think I've actually seen Coulter on tv in years, and I've been watching the news more often in recent years, not less. Back when I would flip through channels, it seems like I would find her giving an interview every week. So maybe the real problem with how she isn't selling as many books as she used to (though any poet would be desperately happy if they sold even half of what she sells now) is that she lacks visibility in the media.
The other thing I might mention is that Coulter's books all appear, at least in their descriptions and cover design, to be similar. (I haven't read them.) They might not actually be about the same thing, but the appear to be about the same thing. Some potential readers may assume, perhaps erroneously, that her books are all about the same thing, and thus don't buy them. A large portion of book selling is based on the idea of making books that are close enough to other popular things as to be familiar and safe but being different enough that people want to see what's new. A large portion of her sales may be more about marketing than her.