Ashes of Honor (October Daye #6) by Seanan McGuire
Synopsis from GoodReads (may contain series spoilers!):
It’s been almost a year since October “Toby” Daye averted a war, gave up a county, and suffered personal losses that have left her wishing for a good day’s sleep. She’s tried to focus on her responsibilities—training Quentin, upholding her position as Sylvester’s knight, and paying the bills—but she can’t help feeling like her world is crumbling around her, and her increasingly reckless behavior is beginning to worry even her staunchest supporters.
To make matters worse, Toby’s just been asked to find another missing child…only this time it’s the changeling daughter of her fellow knight, Etienne, who didn’t even know he was a father until the girl went missing. Her name is Chelsea. She’s a teleporter, like her father. She’s also the kind of changeling the old stories warn about, the ones with all the strength and none of the control. She’s opening doors that were never meant to be opened, releasing dangers that were sealed away centuries before—and there’s a good chance she could destroy Faerie if she isn’t stopped.
Now Toby must find Chelsea before time runs out, racing against an unknown deadline and through unknown worlds as she and her allies try to avert disaster. But danger is also stirring in the Court of Cats, and Tybalt may need Toby’s help with the biggest challenge he’s ever faced.
Toby thought the last year was bad. She has no idea.