For any novel to become a classic, there must be a universality to it. That is, the theme must have a lesson for all times and the plot must be one that appeals to many and have a timelessness to it. There also must be a verisimitude to the novel. "Les Miserables" meets these criteria; Victor Hugo's attention to detail is marvelous and he portrays events accurately. Certainly great attention to detail is given in the first book as Hugo writes of the Battle of Waterloo; he describes in great detail the weather problems, the anxiety of the Duke of Wellington at being outnumbered at Waterloo, the false reports to Napolean that his cavalry could make their way through this mud and gullies to the other side.
The description of the sewers and gamins around the seamier parts of town are, indeed, realistic and accurate, as well, as is the youthful revolution in the streets of Paris in which Marius becomes involved.