The Demon's Lexicon (The Demon's Lexicon Trilogy, #1) - Sarah Rees Brennan
Nick is dark, quiet, and exudes danger. Alan is friendly, open, and puts others before himself. The brothers back each other up no matter what. For years they've been hiding from Black Arthur and his minions, trying to keep their mother and the charm she stole from the master magician safe. They've been constantly on the look out for signs they've been found and ready to move at a moments notice. This time is no different, except for the brother and sister that come to them for help.

Nick doesn't want anything to do with Mae and Jamie, but Alan can't turn them away. Jamie has been marked by a demon and needs help removing the mark or he'll become possessed. In the process of trying to help, Alan also becomes marked. After some investigation they find out both Alan and Jamie's marks are a result of The Obsidian Circle and the only way to save them both is to kill the circle's leader - Black Arthur.

Sarah Rees Brennan has created an interesting story full of mystery, suspense, questions of loyalty and friendship, and what it really means to be family. Nick, not necessarily the nicest guy, becomes a character you can't help but love. You'll be ready to move on to the second book in the trilogy after you finish DEMON'S LEXICON.