I went into this project well-prepared. I knew that only 1 in 3 chapters would contain actual plot, and that there was absolutely no way to tell whether a given one was a plot-chapter without reading the entire thing.
In particular, I was warned ahead of time that I would learn more details than I ever wanted to know about the sewers of Paris. Also that there was a giant chunk of the book devoted to the Battle of Waterloo for some reason.
While historically very important, Waterloo occurred 40 years before the plot of the book. There are 19 chapters about the battle, and only a tiny little nugget of plot at the very end of the very last chapter. I knew this before I started reading Les Misérables. I knew that at some point I was going to have to abandon the plot and read a random book about the Battle of Waterloo. I often read several books at the same time, I was ok with this. I knew I needed to not be emotionally involved in the plot because at some point it would go on hiatus for a good long time.
And then I was reading intently along as Fantine died and Valjean escaped from jail and was on his way to rescue Cosette, when suddenly it snuck up on me.