Magic for Nothing by Seanan McGuire

Improbable, adjective:
1. Not very likely to happen; not probable.
2. Probably not a very good idea anyway.
3. See also “bad plan.”

As the youngest of the three Price children, Antimony is used to people not expecting much from her. She’s been happy playing roller derby and hanging out with her cousins, leaving the globe-trotting to her older siblings while she stays at home and tries to decide what she wants to do with her life. She always knew that one day, things would have to change. She didn’t think they’d change so fast.

Annie’s expectations keep getting shattered. She didn’t expect Verity to declare war on the Covenant of St. George on live television. She didn’t expect the Covenant to take her sister’s threat seriously. And she definitely didn’t expect to be packed off to London to infiltrate the Covenant from the inside…but as the only Price in her generation without a strong resemblance to the rest of the family, she’s the perfect choice to play spy. They need to know what’s coming. Their lives may depend on it.

But Annie has some secrets of her own, like the fact that she’s started setting things on fire when she touches them, and has no idea how to control it. Now she’s headed halfway around the world, into the den of the enemy, where blowing her cover could get her killed. She’s pretty sure things can’t get much worse.

Antimony Price is about to learn just how wrong it’s possible for one cryptozoologist to be.