This book might be a 500-page tragedy, but make no mistake: it's hilarious. And by hilarious, we don't mean gargoyle slapstick: the fact is that Hugo is totally tongue-in-cheek about just about everything. If you find yourself reading this novel and thinking: "Wow, the Paris of the 15th century was really messed up," then you're on the right track.
Don't be fooled into thinking that Hugo is being serious when he says something like: "It was obvious that the bell ringer had been engaged to serve the Archdeacon for a specific time, at the expiration of which he would be sure to steal his soul by way of payment" (IV.V.9). That "it was obvious" thing is key to getting the sarcasm of the punch line. It's a joke, because nothing about that statement is obvious at all. Lines like this show us that the narrator is just about the last person on earth to buy into the superstitions of medieval Parisians.
Sometimes when we read a novel that's a couple of centuries old, it's harder for us to tell when a person is being a stuffy product of their time and when they're trying to make fun of something. That's totally okay. But have faith that this narrator knows crazy when he sees it. Take a passage like this one:
Now this auditor was deaf. A small defect in an auditor. Master Florian, nevertheless, gave judgment without appeal, and very consistently too. It is most certain that it is quite enough for a judge to appear to listen, and this condition, the only essential one for strict justice, the venerable auditor fulfilled even more exactly since no noise could distract him. (VI.I.8).
It's hard to not see this passage as scathingly critical. But it's funny, too, because the courtroom becomes its own kind of comedic play where the characters find themselves in ludicrous situations—except in this case, they actually have to suffer real consequences. Call it dark humor.
Likewise, here the narrator pokes fun at Gringoire's play:
[…]at the same time bestowing on the honest audience as many maxims and aphorisms as could in those days have been picked up at the Faculty of Arts, in the examinations; there were sophisms, arguments, figures of speech, and other wordplay by which masters acquired their caps and their degrees.
All this was really very beautiful. (I.II.71-72)
The sarcasm in this one might be a little trickier to spot, but basically the joke is that Gringoire seriously overdoes it with the figures of speech, to the point where he sounds like a rhetoric exam. (They used to have those back in the day.) But is the narrator isn't just going to come out and say that Gringoire is a terrible playwright; it wouldn't be very funny that way. By pretending to go along with it, the narrator is mimicking Gringoire's own delusions about his amazing literary abilities. That adds a whole layer of funny to scene that wouldn't be there if the narrator just told us everything straight all the time.
And you thought this book was all work and no play.