In this third installment of The Mortal Instruments trilogy-thing-that's-over-three-books-now-and-will-probably-be-six-books-I-think?, we get the Epic Battle that we'd all been waiting for ever since we learned that Valentine (Clary's and Jace's father, who is plotting against the good guys) was still alive, back in the first book. In this novel, however, our protagonists spend very little time in New York City; rather, the bulk of the action takes place in Alicante, the (magical?) capital of Idris, the Shadowhunter country.
Also, we learn new things about Clary's past and relatives (some of which were hinted at in the previous books, and some of which WOAH THAT WAS UNEXPECTED. Not unwelcome, but very unexpected.), and we finally learn about the third Mortal Instrument (remember, the first book was about the Mortal Cup? And the second one involved the Mortal Sword? Well, astute readers will finally have their suspicions confirmed/blasted to pieces about the mirror. Wait, that should have been the Mirror - you can't forget the capitalization.).
As for who is gracing the cover art - I'll let you figure it out. And no, it's neither Simon nor Alec (yep, it's a new character - and I won't say anything more because SPOILERS!)
So yeah, I really liked this book, as well as the overall series; it's a fun, easy, action-packed and emotionally rollercoaster-ish read.