City of Ashes is the 2nd book in the Mortal Instruments series by Cassandra Clare. This is a young adult series. Spoilers to follow, especially if you haven't read the first book!
Here's the synopsis from Barnes and Noble:
Clary Fray just wishes that her life would go back to normal. But what's normal when you're a demon-slaying Shadowhunter, your mother is in a magically induced coma, and you can suddenly see Downworlders like werewolves, vampires, and faeries? If Clary left the world of the Shadowhunters behind, it would mean more time with her best friend, Simon, who's becoming more than a friend. But the Shadowhunting world isn't ready to let her go -- especially her handsome, infuriating, newfound brother, Jace. And Clary's only chance to help her mother is to track down rogue Shadowhunter Valentine, who is probably insane, certainly evil -- and also her father.
To complicate matters, someone in New York City is murdering Downworlder children. Is Valentine behind the killings -- and if he is, what is he trying to do? When the second of the Mortal Instruments, the Soul-Sword, is stolen, the terrifying Inquisitor arrives to investigate and zooms right in on Jace. How can Clary stop Valentine if Jace is willing to betray everything he believes in to help their father?
As with the first book, I had a hard time putting this down. The tension between Jace and Clary was intense in this book. I kept waiting for them to somehow find out that they aren't brother and sister. Sigh. Lots of twists in this one, and a lot of action. I was afraid there might be several 'main' characters dead after the battle on Valentine's ship! (Luckily they are safe for now!) Of course, Valentine is still out there somewhere. I haven't read any info about the third book yet, but I'm hoping to get it in the next week or two (it's a lot less expensive online, so I have to wait for it to ship!). I'm anxious to find out what happens, and to see how things are wrapped up (it's a trilogy, right?). Now I just have to avoid reading everyone's reviews of the third book - I really hate spoilers!! :)