The Demon's Covenant The Demon's Lexicon Trilogy, Book 2.

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 tha...