When people talk about books, they generally mean books, not audio books. Yes, the narrator of an audio book can choose to use a certain accent, but that’s not necessarily what the author intended. In this case, the author didn’t make any suggestion that the characters sound like that, it was a choice made by Ms. Pike or by the audio book director.
You can’t say that the characters in the book have that accent, all you can say is that in the audio book they’re given that accent.