I have a bit of an ambivalent relationship with books by this author - on the one hand, I absolutely devoured A Darker Shade of Magic and its sequel, A Gathering of Shadows, but I couldn't even finish Vicious. So I approached This Savage Song with some uncertainty and, for the first 100 pages at least, I thought this was going to be another DNF (which is why it's got 3 stars from me instead of 4).
This Savage Song is more traditional dystopian urban fantasy than anything else Schwab has written, a world where there are 3 kinds of monsters and everyone knows about them - the worst, the sunai, are created when something terrible happens (a bombing, a school shooting, a mass suicide) and we learn through the progress of the book that one of our main characters (August Flynn) is one of these, a creature which can use music to feed on humans but only if they're 'sinners'. Anyway, in short it's a post-apocalyptic world where people live in a city-state called Verity in what used to be the US, a city which is divided between the followers of Callum Harker, who has 'control' of some of the monsters, and those of Henry Flynn and his children.
Our other main character is Callum Harker's daughter, Kate, who we first meet setting fire to the chapel at her current school in a (successful) attempt to get kicked out and sent home to Verity. It's fair to say that Kate is quite unpleasant at times, which is pretty much in keeping with how this author writes both her male and female characters fairly even-handedly (a plus point in my opinion) and is desperate for her father's attention. When she starts school in Verity, August is sent to spy on her by attending the same school, which would be fine if one of Harker's monsters wasn't trying to start a war by framing August for Kate's murder. As a result, the two of them go on the run together and hijinks ensue.
By the way, if you're looking for a romantic sub-plot, This Savage Song is not the book for you. I've given it three stars because I can't see myself reading it again, and also because it drags at the start in a way that almost had me taking it back to the library unfinished, but I'll definitely keep an eye out for the sequel (as yet untitled but due out next year).