The sixth October Daye book. Spoilers ahead for the earlier ones.
After a dramatic opening with some help from Tybalt, Toby gets home to find surprising company: Etienne, another knight in Sylvester's service, but while Toby was given her knighthood because of her PI work, Etienne is an actual knight, and old-fashioned.
Which why she is more surprised when he confesses to having fathered a changeling in the unpleasant times that started the whole series. Her mother had hid the girl, Chelsea, from him, until just now, when he got an accusatory phone call demanding her back. She's vanished. He hires Toby to find her.
The course of the investigation turns up a fantastic creature that crushes Toby's car, and they realize that Chelsea is one of those changelings who lack not the powers but the limits of their parents, and her enormous ability to teleport could endanger the world.
Unraveling it all involves heading into regions that have been cut off for centuries, a rose, a ruby, a declaration of love motivated by the real possibility that they will not live to another chance, Jin yelling at people ignoring her medical advice, and really bad tapestries.