Lady midnight - Cassandra Clare (The Dark Artifices #1)

In a kingdom by the sea...

In a secret world where half-angel warriors are sworn to fight demons, parabatai is a sacred word. A parabatai is your partner in battle. A parabatai is your best friend. Parabatai can be everything to each other — but they can never fall in love.  

Emma Carstairs is a warrior, a Shadowhunter. She lives for battle. Alongside her parabatai, Julian Blackthorn, she patrols Los Angeles, where vampires party on the Sunset Strip, and faeries — the most powerful of supernatural creatures — teeter on the edge of war with Shadowhunters. When the bodies of humans and faeries are found murdered in the same way Emma’s parents were when she was a child, an uneasy alliance is formed. This is Emma’s chance for revenge and Julian’s chance to get back his brother Mark, who is being held by the faerie Courts. All they have to do is solve the murders within two weeks... and before the murderer targets them.  

Their search takes Emma from sea caves full of sorcery to a dark lottery where death is dispensed. And each clue she unravels uncovers more secrets. What has Julian been hiding from her all these years? Why does Shadowhunter Law forbid parabatai to fall in love? Who really killed her parents — and can she bear to know the truth?