Being the third book of the Gemma Doyle trilogy by Libba Bray. The title is taken from the W. B. Yeats poem "The Rose of Battle", which sounds a lot cooler than the title does by itself. (Well, the excerpt of it in the epigraph does, at least. I haven't read the whole thing.) The first and second books are A Great and Terrible Beauty and Rebel Angels.
It's sweet and it's far. What more do you need to know?*
I found all three books in the trilogy difficult to read straight through. Gemma Doyle starts off the very first book (A Great and Terrible Beauty) by lying to teachers in order to ingratiate herself to other students at her new finishing school, and continues on with similar selfish behavior. In one of Aesop's fables, Something Bad would therefore happen, but the unfortunate events in these books are often ascribable to fate rather than consequence. Although she can also be tender and caring, it seems that she is that way (without ulterior motives) to only a few people, her closest friends and her father, until partway through the third book.
This is a Victorian Gothic, a strange hybrid of finishing schools and parties in turn-of-the-century England juxtaposed with sinister spirits and magic in "the realms." Gemma and her friends are least bearable when they have magic at their disposal: they seem to use it at every whim, with no apparent self-control. Part of the tension hanging over the series is caused by the assumption one would make that walking around like you own the world and doing whatever you want will cause trouble, but as I mentioned, most of the trouble seems to come from other causes, although it was aggravated by Gemma's reluctance to deal with it promptly. Instead of keeping the numerous promises that she makes (and breaks) so easily, she entertains illusion and glamour in a sort of fairyland. Her friends are, admittedly, somewhat culpable in her behavior, but most of her growth as a character seems to come fairly quickly near the end. (At least, that is my recollection, although I read the first two books a couple months ago.)
In the end, if you like dresses and games and vicious cliques and intrigue and sinister magic, you may like this trilogy. I did end up enjoying the third book somewhat more than the first two, because the characters were no longer in stasis. However, I probably wouldn't read it again.
* Yes, I more or less stole, err, borrowed this line from Maureen Johnson, who has at the linked location a short video interview with Libba Bray. Short and funny.