The Undead Pool — Kim Harrison

Witch and day-walking demon Rachel Morgan has managed to save the demonic ever-after from shrinking, but at a high cost. Now, strange magic is attacking Cincinnati and the Hollows, causing spells to backfire or go horribly wrong, and the truce between the Inderlander and human races is shattering. Rachel must stop this dark necromancy before an all-out supernatural war breaks out.

Rachel knows of only one weapon to ensure the peace: ancient elven wild magic, which carries its own perils. And no one knows better than Rachel that no good deed goes unpunished...