I read this book in two days and I really enjoyed it, I thought it was a really good end to the series and was probably my favourite in the series along with Clockwork Prince.
It is told from varying narrators like the previous books in the series, but this one probably has the least narration from Tessa out of the three which I like because just like The Mortal Instruments I start to care just as much about the side characters as much as the main ones. My biggest problem with this book was that the first third was really great and fun, the last third was really emotional and concluding, but the middle third was really boring and I found it really tedious to get through. My cousin, who had read these before me, said that she disliked this book because the end was really weird, and because this book ties in with City of Heavenly Fire a large plot point at the end goes unexplained and I can totally understand how having read this without reading City of Heavenly Fire first could turn you off the ending. The thing that was most annoying to me about this book was that the friendship between Sophie and Tessa had become like my favourite thing and all I wanted was for Sophie to tell Tessa she was engaged, but I was deprived of that beautiful moment for like no reason, the book was long enough, a few more pages couldn't have hurt. I would recommend this series to fans of the YA paranormal genre, but I'm pretty sure if you like this genre you've probably already read it.
The book starts with Gabriel Lightwood coming to the Institute to tell them that his father's demon pox has turned him into a demon, a giant worm to be specific, and he's tearing people apart at their house. Everyone goes to fight him together and after they kill him they return to the house with Gabriel, who was blackmailed by the Consul to spy on Charlotte and send him some evidence that she is unfit to run the institute. Jessamine is being returned to the institute from the silent city but when the silent brothers bring her, Mrs Black and an army of automatons also come in the gates and they kill Jessamine and take Tessa. Jem collapses after the battle and when Will goes to give him more yin fen he realizes there is next to none left because Jem needed more and more to keep his strength up recently, and Mortmain has bought up all the supplies so they can't buy any more. Will then works out where Mortmain is, and he wants to go rescue Tessa but he feels he can't leave Jem. Magnus convinces him that it is what Jem would want even though he doesn't know of Will's feelings for her and Jem wakes up and hears them. He talks to Will about loving Tessa and doesn't hate him for it, he insists he goes to rescue her and they say their goodbyes. Will then travels to his family's old house in Wales where Mortmain is keeping Tessa, on his journey there he feels through his parabatai mark that Jem has died but he continues on. When he finds Tessa, trapped in a cave, she has already been forced to change into Mortmain's father and give him the spell that binds demons and automatons so they can think on their own. The only way to defeat the automatons now is to kill Mortmain. Will and Tessa have sex, and when Magnus finds them the next morning because he helped Henry perfect the portal and they all travelled through it together he helps them get ready for the battle. The consul has a grudge against Charlotte because she's a woman and would not send any other Shadowhunters to her aid but three Silent Brothers are fighting with them when Tessa and Will arrive and they see that one of the brothers is Jem. Tessa is then taken by Mortmain, but before he can take her away she changes into the angel Ithuriel, who's spirit was trapped inside her necklace protecting her for her whole life, and kills him. Jem then tells says his final goodbyes to Tessa and Will and gives them his blessing to be together, Charlotte becomes Consul and says that Will will take over running the institute, Magnus takes Church with him to New York, Sophie ascends and becomes a Shhadowhunter and is engaged to Gideon, and Gabriel and Cecily start dating. The epilogue takes place in 2008 for Tessa and Jem's yearly meeting and after Tessa explains of her years of happy marriage to Will, Jem arrives, but not as a Silent Brother anymore, he has been returned to the normal Jem he was, but not sick anymore and he and Tessa kiss.