In the case of a novel, or even certain text books, the author relies on the reader not jumping ahead. Especially murder mysteries and those kind of genres. There are artistic reasons for that, and it can wreck the work.
In my experience, AI summaries often miss points or misrepresent work. There is a human element to reading a well written novel. An AI will miss some of the subtleties and references.
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.
In my experience, AI summaries often miss points or misrepresent work. There is a human element to reading a well written novel. An AI will miss some of the subtleties and references.