How much of any novel that you’ve read more than five years ago can you recall? Probably in some cases you have a strong memory of the plot, maybe some lines of dialogue (though these may well not be remembered verbatim…we tend to make small changes, as in the way “quotes” like “Play it again, Sam,” and “Alas, poor Yorick, I knew him well,” replace the originals). Maybe we remember specific scenes, though again memory isn’t quite reliable enough to ensure the we remember them entirely accurately. (A search through ERIC didn’t find terribly consistent results on this, nor much of anything focused on novel reading, so I’ll just go by experience and intuition here, but I’m pretty sure our ability to recall scenes in novels is no better than our ability to recall scenes from real life, about which there are mountains of studies on our unreliability.)
It’s not the purpose of a novel, of course, to forever imprint itself in our minds, nor is the value of the novel found in what we remember of it later. It could be that we have no memory at all of reading a book that had some profound influence upon us (I remember once being reminded of a book I’d read as a young teenager and had not thought of in many years, and realizing that it had lead me towards many other books, and had perhaps shaped my sense of story, or perhaps merely answered to what I would have valued in a story.)
If it were the purpose of a novel to do so, a summary of the novel might well be more effective than the novel itself. I could fairly easily pass a test on some classic I’d never read by just reading over its Wikipedia page and perhaps a few other sources the night before the exam. So if there’s some particular value in novel reading (and I don’t think there needs to be!) it can’t be in what one remembers of the novel, or the novel would be no more valuable than a summary or ‘Cliff’s Notes’ version. Which, as a philosopher pursuing the question, I can’t rule out. Perhaps we should all just read the Cliff’s Notes. It’s not an unreasonable hypothesis, anyway.
Though I’d rather look for some value in reading the novel itself (not to dismiss the value of reading summaries.)
But it raises an interesting possibility: perhaps being memorable is not an absolute mark of the value of a novel, nor is being forgettable an absolute mark of its lack of value. Again, the novel I read as a youngster that I was reminded of might be only one of many that I’ve forgotten that was, nonetheless, valuable in some way.
Maybe the value is merely in the entertainment and distraction that it provides while being read. I don’t want to dismiss the very positive value of this; I think being distracted can be a wonderful thing. Certainly in an unpleasant life, a few moments of distraction are a gift not to be taken lightly.
But if it’s not simply the entertainment value, nor what one remembers of the novel (even if those are important elements of value) what else is the value of reading?
One possibility: It might be a combination of these things. In other words, while novels may convey ideas or instill memories, they do so with a particular style and in a particular manner that is inherently valuable because they allow for forms of aesthetic appreciation.
Another: the summary, by being brief, leaves out one of the elements that makes the novel what it is, the extension through time, and the slowing down of the story. It’s been said that the plot of any novel can usually be summarized in a page or so (I can’t remember who said that, or the exact quote, but I want to make clear this is not my insight) and that the majority of the text is doing something besides plot. Part of what it’s doing is slowing down the revelation of plot! Extension has its pleasures.
The first possibility and the second may pertain to greater and lesser extent to different sorts of novels, as well. If the emphasis is on beautiful language and “literary” qualities, the conveyance of ideas with style may be a main source of value. If the emphasis is on enjoyment of the plot, the second possibility may pertain.
And this points to what some have said is a difference between “literary” and genre works: that genre works focus more on plot. I don’t know that I buy that difference, but there may be a genre difference between plot-oriented works and works that are less concerned with plot.
And one could claim that genre works are lesser because they’re plot oriented, and plot can be summarized, whereas the literary elements of the text are lost in summary: noting that the Nabokov’s language is complicated and rhythmic and etc. is not the same thing as reading complicated, rhythmic language.
But! It’s also not the same thing to read a summary of a plot and to feel the plot stretched out and extended, information withheld for (if the author is careful) just the right amount of time. The plot doled out in proper pacing, which is what fiction that’s read for pleasure is supposed to do well, is also lost in summary. So perhaps some recapturing of the value of the story-oriented book can be had if we understand the way in which the pleasures of the plot are necessarily delayed, and how that delay is itself an element of aesthetic craft.
Popular Metaphysics 
One of the main values of reading is that it’s still, evolutionarily speaking, a new thing, and thus not a natural thing for any person to do. Each brain has to organize itself to be able to apprehend and interpret written language and the act of reading results in dramatic physical changes to the brain. Each book we read alters us. A person can be made smarter or dumber depending upon the books he/she chooses to read. A more complicated text, fully read and considered, results in a brain marginally more able to comprehend and consider complexities. Formulaic genre text offers little to the brain and merely reinforces established connections and patterns. Which, I guess, could be okay. Sometimes.