The demon's covenant - Sarah Rees Brennan

Mae Crawford always thought she was in control. Now she's learned that her little brother Jamie is a magician and Nick, the boy she'd set her heart on, has an even darker secret. Mae's whole world has spun out of control, and it's only going to get worse. When she realises that Jamie has been meeting secretly with the new leader of the Obsidian Circle and that Gerald wants him to join the magicians, she's not sure how to stop Jamie doing just that. Calling in Nick and Alan as reinforcements only leads to a more desperate conflict because Gerald has a plan to bring Nick down - by using Alan to spring a deadly trap. With those around her torn between divided loyalties and Mae herself torn between her feelings for two very different boys, she sees a chance to save them all - but it means approaching the mysterious and dangerous Goblin Market alone...