For anyone who has seen the Disney movie you will need to cast the voices out of your head because the book is very different.
It is indeed dark, with themes of obsession, jealousy, murder and death, Disney has to adapt the story to suit younger eyes.
The characters are structured, but each of them have their flaws. Esmerelda is seen as pure but her pure sexuality tempts Phebus, captain of the guard, which then causes Frollo to act murderously. Frollo orders Quasimodo to do his evil bidding because he is the master and Quaismodo is a "poor excuse for a human being."
The key scenes in the book are heart-wrenching. Esmerelda shows Quaismodo kindness even though he brutally tried to kidnap her the night before, Frollo's obsession makes your skin crawl and the ending, is the most touching.
Victor Hugo used the central backdrop of Notre Dame to great effect. By hiding Quasimodo in there for sanctuary and later Esmerelda, as she escapes Frollo, it shows the safety of the church, but it is not enough to have faith in religion. Because the consequences of their actions outweigh the power of Notre Dame.