From an artistic point of view, yes it is. It's a bit like doing a crossword with the answers in big letters next to it... It destroys the point.

I agree but for some reason, there are people who enjoy doing that. I think they should be allowed to do as they like.

In any case, Amazon claims this feature is spoiler-free and that would be easy to implement. It likely works by feeding the book into an LLM context, and they could simply feed in the portion you've already read.