Book Nine of the Dresden Files

Someone is targeting the city's magic practitioners, the members of the supernatural underclass who don't possess enough power to become full-fledged wizards. Many have vanished. Others appear to be victims of suicide. But the murderer has left a calling card at one of the crime scenes--a message for Harry Dresden, referencing the book of Exodus and the killing of witches.

Harry sets out to find the killer before he can strike again, but his investigation turns up evidence pointing to the one suspect he cannot possibly believe guilty: his half brother, Thomas. Determined to bring the real murderer to justice and clear his brother's name, Harry attracts the attention of the White Court of vampires, becoming embroiled in a power struggle that renders him outnumbered, outclassed, and dangerously susceptible to temptation.

Harry knows that if he screws this one up, a lot of people will die--and one of them will be his brother.