Ever After (Hollows Book 11) by Kim Harrison

The demonic realm, the ever-after, is shrinking and if it disappears, so does magic.  And it happens that it is witch and day-walking demon Rachel Morgan's fault.

After causing a rip in the ley line a chain of events was set off that could cause a world apocalypse. Rachel must find a way to save the people she cares about, keep the balance between the human and demon worlds, and oh, and if she fails, her life is forfeit.