Yes, there are definitely some good critical writings, the first that comes to mind is Mario Vargas Llosa's The Temptation of the Impossible, which I recommend since it's also just a good read. There's also Kathryn Grossman's Les Misérables: Conversion, Revolution, Redemption and Victor Brombert's Victor Hugo and the Visionary Novel and if you can read French, there's André Brochu's Hugo — Amour / Crime / Révolution.
I'd also recommend Graham Robb's Victor Hugo biography. It's not strictly about Les Mis, of course, but it's great and also Graham Robb is awesome. A lot of Les Mis is drawn from Hugo's personal experiences and political allegiances (and shifting thereof) so if you want to understand where he's coming from with certain story elements, his biography is a great place to start.
"Quand vous aurez besoin de Bahorel, capitaine, Bahorel est là! Je sais faire trébucher tous les chevaux du garde-corps avec une ficelle... Rien qu'une petite ficelle. Enfin, pensez à Bahorel du Café Musain!"