I was once reminded of a movie I had watched two weeks prior and realized I hadn't thought about the movie once since leaving the theater. It occurred to me that this is a good proxy for whether I think the movie or book was worth-while.

Years after, I'm not surprised your recall of details from War and Peace has faded, but I don't think that's necessary for "deep insights". In the 18th and 19th centuries, people thought that reading novels provided exercise for the "moral imagination". Reading a novel or essays that provide an opportunity to turn things around in your mind in a way that yields interesting insights leaves you with something more durable than a plot synopsis.