Series: October daye

The October Daye series, by Seanan McGuire. They're contemporary fantasy, with fairyland strongly overlaying the modern world, though most people are unaware of it. These are set in the San Francisco Bay area.

October is a half-breed, but a knight of one of the elven courts. She's a private investigator, and the only one with elven lineage so the only one they can trust (or who knowns anything about them). This also makes her weaker than most of the elves. That hardly matters, though, since her cases often pit her against extreme upper levels of the elven hierarchy.