White Night — Jim Butcher

Someone is targeting Chicago's magic practitioners, the members of the supernatural underclass who don't possess enough power to become full-fledged wizards. Some have vanished. Others appear to be victims of suicide. But now the culprit has left a calling card at one of the crime scenes-- a message for Harry Dresden.

Harry sets out to find the killer, but his investigation turns up evidence pointing to the one suspect he cannot possibly believe guilty: his half brother, Thomas. To clear his brother's name, Harry rushes into a supernatural power struggle that renders him outnumbered, outclassed, and dangerously susceptible to temptation.

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