I started shamefully read this on my kindle about two minutes after I finished the second book. It is much, much better than the second one - the story line seems better thought out and is explained better - in the previous installment I felt a bit like it went around in circles but didn't actually make itself very clear.
In the final book of the trilogy the Clave must decide whether to give in to Valentine's threat of opening up the demon world to destroy all those who are not 'pure' Shadowhunters - those who associate with werewolves, vampires or warlocks, and those who have corrupted the original mission of Shadowhunters (to destroy demons). He already has a magic sword and a magic cup, all he needs is to find the magic mirror and he can summon the original angel to grant him his wish - its up to the usual gang to try and stop him.
There are new characters in this book, some of which prove very important to the story's outcome! Whilst it is obviously a trashy teen romance that we all hate to love, this book definitely has a good story and is told really well. The whole romance thing between Clarry and Jace is obviously constantly at the heart of the book but I feel it is written about in a more tolerable manner with less of the awkward cringing awfulness that always ruins a fantastic story!
I was also worried that this book would end with a huge cliffhanger that would force me into reading the next "City of..." which would have really annoyed me (mainly because I was so hooked whilst reading this one that I stayed up all night reading it and never got any sleep...), but fortunately it has a definitive (happy!) ending so you can rest in peace knowing your somewhat embarrassing foray into the Shadowhunter teen fiction realm is at an end.