Just as duplicating a fragment can be legal, duplicating any fragment on demand is not. Rephrasing a passage might be legal, but rephrasing any passage on demand might not.
https://1minutebook.com/are-book-summaries-legal/
Fair use is a defense often cited in those cases but it's just that: a defense. Cliff Notes are often cited here but they actually license the content in many cases.
> If you want to write a summary of any novel, without quoting from it, you are free to do it
> Copyright does not protect ideas, only a particular expression of those ideas
> You would likely get in trouble only if your summary contained long excerpts directly from the book
> As long as you do not quote directly from the book, or copy any of the content, then writing a unique summary is not illegal. You can mention the title, you can even quote sentences from the book as long as they are cited, you just can’t reproduce chunks of the content
etc
(I'm also not sure whether this article is just blogspam or itself AI generated)