A Local Habitation by Seanan McGuire

Sequel to Rosemary and Rue.

Toby gets a call from her lord Sylvester, who needs her PI skills.  His niece January hasn't been answering his calls.  All the more complicated in that his niece is a Countess, and her county lies between his duchy and a rival one. . . and the terrioritiality of the fae means he won't be able to get her help easily, without its being taken as an act of war.

She gets there, tracks down January, finds a small array of fae, of different races, and discovers that January thinks Sylvester hasn't been answering her calls, and that she's definitely hiding something.

And getting to the bottom of this, involves a password "Do your homework", a tank of hippocampi, dead bodies, a brother and sister who never seem to be together, a very strange adoptive daughter who can teleport, Toby's asking her last question from the Ludeiag (dangerous, that, since now that she's no longer in debt to Toby, she can kill her), the mixes of fairy blood, a four-tailed Kitsune, and an attempt to save Fairy from itself.  Among other interesting developments.