This book is presented as being written by a human. (In this case, a human I happen to know, which would add another dimension to my complaint, so let's imagine that weren't the case.) Maybe someday we'll live in a world where sticking a person's name at the top of a book doesn't carry the implication that they wrote the words in the book. That would make me sad for other reasons, but in any event, we don't live in that world right now, so for now, if you put a person's name on your book, you are claiming that that person wrote the book. If that's not true, you are lying, and like most people, I don't like being lied to.
I'm imagining something more like the way we treat movie directors. When I see Michael Bay vs. Quentin Tarantino, I don't think that those people wrote the film, or set up every camera shot in minute detail, or designed the costumes, etc. but I know that they had a hand in arranging for all of that and it will all ultimately come through with their unique stamp on it.
Not even an AI thing, we don't. Ghost writing has been going on for centuries.
I don't like that! I agree it's dishonest and I wish we lived in a world where having been exposed employing a ghostwriter is a disgrace no public figure can come back from. I don't like that AI has made it worse, either. At least with ghost writing there are some contexts (politicians, celebrity bios) where you might suspect it, but in most you won't.
But, we do live in that world right now.