The Demon's Lexicon, by Sarah Rees Brennan

Nick and Alan try to live normal lives, but it's difficult when they must repeatedly fight or flee from the magicians who hunt their family.  This little-known subculture of magic users increase their power by calling demons into the world whose prime desire is to possess a human body.  What starts out as a straight-forward good guys versus supernatural bad guys tale becomes much more uncertain when Nick begins to question his brother's motives and honesty.

Readers of Cassandra Clare's Mortal Instruments series will love this book, and dark, brooding Nick in particular.