White Night (The Dresden Files #9) by Jim Butcher

The inspiration for the Sci Fi channel television series

In Chicago, someone has been killing practitioners of magic, those incapable of becoming full-fledged wizards. Shockingly, all the evidence points to Harry Dresden's half-brother, Thomas, as the murderer.

Determined to clear his sibling's name, Harry uncovers a conspiracy within the White Council of Wizards that threatens not only him, but his nearest and dearest, too...