As far as musical theater and the most recent adaptation of the story go, I'm actually not a huge Les Mis fan. I'm going to have to say that the book is my favorite of any version of the tale by far, but whether that's because I found its differences from the musical very interesting or if it would have been as good on its own I cannot say. However, no matter, for I was endlessly fascinated by this much fuller tale.
I did not realize how different the book would be, but because the first 70 or so pages follow the Bishop (rather than Jean Valjean), I quickly gathered that this would be very different. Although the section on the Bishop (or the sections of French history or any of the many unnecessarily long and drawn out expositional portions) was often uninteresting, overall I was impressed with the amount of detail that the novel includes. There's so much more to be known about each of the characters, even simple things like the fact that Gavroche is a Thenardier, and I really appreciate the nuances the book conveys that the musical simply has no time to include. Some things, like Javert's fixation on Valjean, still never became clear to me, but other things, like Cosette's relationship with Marius made sense with context.
Plus, I was rather impressed with the writing. For a novel with a plot so integral to the work as a whole, scenes of action never become more important than such beautiful writing"
“You can give without loving, but you can never love without giving. The great acts of love are done by those who are habitually performing small acts of kindness. We pardon to the extent that we love. Love is knowing that even when you are alone, you will never be lonely again. & great happiness of life is the conviction that we are loved. Loved for ourselves. & even loved in spite of ourselves.”*
*The quote formatting is janky but that's what's on goodreads so I'm going to go with it.