Blood Bound By Patricia Briggs

Under the rule of science, there are no witch burnings allowed, no water trials or public lynchings. In return, the average law-abiding, solid citizen has little to worry about from the things that go bump in the night. Sometimes I wish I was an average citizen...

Mechanic Mercy Thompson has friends in low places-and in dark ones. And now she owes one of them a favor. Since she can shapeshift at will, she agrees to act as some extra muscle when her vampire friend Stefan goes to deliver a message to another of his kind.

But this new vampire is hardly ordinary-and neither is the demon inside of him.