A Local Habitation by Seanan McGuire (2010) DAW

October "Toby" Daye is a changeling, the daughter of Amandine of the fae and a mortal man.  Like her mother, she is gifted in blood magic, able to read what has happened to a person through a mere taste of blood.  Toby is the only changeling who has earned knighthood, and she re-earns that position every day, undertaking assignments for her liege, Sylvester, the Duke of the Shadowed Hills. Now Sylvester has asked her to go to the County of Tamed Lightning—otherwise known as Fremont, CA—to make sure that all is well with his niece, Countess January O'Leary, whom he has not been able to contact. It seems like a simple enough assignment—but when dealing with the realm of Faerie nothing is ever as simple as it seems. Toby soon discovers that someone has begun murdering people close to January, whose domain is a buffer between Sylvester's realm and a scheming rival duchy. If Toby can't find the killer soon, she may well become the next victim.