Imaginary Numbers by Seanan McGuire

The ninth book in the fast-paced InCryptid urban fantasy series returns to the mishaps of the Price family, eccentric cryptozoologists who safeguard the world of magical creatures living in secret among humans.

Sarah Zellaby has always been in an interesting position. Adopted into the Price family at a young age, she’s never been able to escape the biological reality of her origins: she’s a cuckoo, a telepathic ambush predator closer akin to a parasitic wasp than a human being. Friend, cousin, mathematician; it’s never been enough to dispel the fear that one day, nature will win out over nurture, and everything will change.

Maybe that time has finally come.

After spending the last several years recuperating in Ohio with her adoptive parents, Sarah is ready to return to the world–and most importantly, to her cousin Artie, with whom she has been head-over-heels in love since childhood. But there are cuckoos everywhere, and when the question of her own survival is weighed against the survival of her family, Sarah’s choices all add up to one inescapable conclusion.

This is war. Cuckoo vs. Price, human vs. cryptid…and not all of them are going to walk away.