Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
This Bitter Language
I know your streets, sweet city,
I know the demons and angels that flock
And roost in your boughs like birds.
I know you, river, as if you flowed through my heart.
I am your warrior daughter.
There are letters made of your body
As a fountain is made of water.
There are languages
Of which you are the blueprint
And as we speak them
The city rises.
-Elka Cloke
Part One:
I believe I am in Hell, therefore I am.
-Arthur Rimbaud
Part Two:
Before me things created were none, save things
Eternal, and eternal I endure.
All hope abandon, ye who enter here.
-Dante, Inferno
Part Three:
Day of wrath, that day of burning,
Seer and Sibyl speak concerning,
All the world to ashes turning.
-Abraham Coles
What's up with the epigraph?
This poem feels like it was written just for this book. We have a reference to a "sweet city," which could be New York City, where Clary and pals live. There are "demons and angels" and a "river," which is where Valentine docks his scary boat. "I am your warrior daughter" could alert us to the fact that Clary is the narrator here, although she's being a little generous calling herself a warrior. "Klutzy" isn't as poetic. "Letters made of your body" refers to the Marks, runes written on a person's body.
In addition the main epigraph, each part begins with a little quote, usually about hell. Considering Valentine wants to summon a horde of demons, thereby creating a temporary hell on Earth these epigraphs do a nice job building tension as the story progresses. Also, the reference to "ashes" in a book called City of Ashes is a nice touch.