I'm not a big fan of these questions. One reason why I'm not a big fan is I don't really think they meet the requirements for good subjective questions. Our subjective questions, such as "what is the meaning of x", work because answers back their claims up: while there are multiple correct answers, what we are (or should be) judging when we vote on answers is the argument the answers provide about why their conclusions are correct. I don't think the same can be said for these "what is a good summary" questions: two people can provide a summary, but a summary, on its own, does not contain an argument about why the summary is correct, so there isn't really an objective way to differentiate between the summaries; people will be just be voting for the summary they like best. In that sense, these questions are very similar to recommendation questions, which we've closed, because they caused so much trouble the last time people tried to make a Stack Exchange site about literature.
To elaborate, here's a hypothetical question: "Summarize The Fellowship of the Ring". Here's one summary:
Twelve friends set out on a journey to defeat an evil ruler, Sauron. Along the way, a member of their party is lost to a fearsome demon. At the end, the remaining friends realize that their goals do not align, and split off into different directions.
How do you evaluate this summary? First, you need to decide whether it has enough detail. Let's say that the question asked for a three sentence summary, and that the summary has enough detail. Then, you need to decide what points are important enough to include in a summary. Perhaps, looking at the summary, you dislike it because it doesn't even mention the ring; there's a very good argument that the ring is the main character of the novel. So you complain about the summary in a comment, and someone responds that, on the contrary, novels are about people and it seems silly to mention some object. And now you've gotten into a debate in the comments. But the thing about the debate is that both sides are equally right. And if there's one thing we know about these subjective debates, it's that they only work when both sides are willing to back up their claims. But comments aren't a good place to back up claims, they're short and hard to see. So include an argument supporting your summary in your answer. But now you're answer is less about providing a summary, and much more of an answer to the related question: what are the most important points in a novel. The take away here is that the question isn't a good subjective question, because it's designed in a way that encourages people to just plop a summary.
Besides, these questions aren't even good or well researched questions: it would be pretty easy to spam the site with a lot of "Give a summary of book x" questions. So in my mind they should be downvoted, even if they aren't closed, because there's no effort involved in asking for a summary.
If people think there's a chance that these questions might add value, I'm willing to give them a shot and leave them open (until they start causing problems). But I'm pretty convinced that these will fail for the same reason recommendation questions will fail: they're low quality, easy to spam the site with, and don't really meet the criteria of a good subjective question. My recommendation is to close these questions.
Further reading: Good Subjective, Bad Subjective