I first tried it to remind me of what happened in a previous book in a series that I was reading. When I realized it was either misstating plot points or straight up hallucinating, I tried it on a bunch more books to amuse myself.
Older classics are of course more accurate, but for newer or less popular books Gemini won't shy away from giving you a summary culled from misinterpreted Reddit threads and Goodreads reviews. It's like getting a secondhand account from someone who talked to another person who had read the book a long time ago. You get the general gist of it, but with some added flavor.
Even if you upload an entire epub of a book, the results aren't stellar. Rather than a Cliffs Note's quality summary, they're pretty sparse or leave out important bits of information. One chapter summary I got back made a point of describing what one of the characters was wearing, even though it had absolutely zero to do with anything else. Yes, that's technically a "summary", but not quite my tempo.
If Google wants to present summaries of websites in anything more than a very, very superficial description, they're going to have to improve their model's ability to understand context and importance. In theory, a novel is a self-contained bundle of text, so pulling accurate information out of it should be straight forward. A website is naturally going to be way more of a challenge.
All that said, I find the AI summaries from Google/Gemini to be quite useful and a time saver, but I know to always double check something if it's at all important.