Hemlock & Silver by T. Kingfisher

From New York Times bestselling and Hugo Award-winning author T. Kingfisher comes Hemlock & Silver, a dark reimagining of “Snow White” steeped in poison, intrigue, and treason of the most magical kind

Healer Anja regularly drinks poison.
Not to die, but to save—seeking cures for those everyone else has given up on.
But a summons from the King interrupts her quiet, herb-obsessed life. His daughter, Snow, is dying, and he hopes Anja’s unorthodox methods can save her.
Aided by a taciturn guard, a narcissistic cat, and a passion for the scientific method, Anja rushes to treat Snow, but nothing seems to work. That is, until she finds a secret world, hidden inside a magic mirror. This dark realm may hold the key to what is making Snow sick.
Or it might be the thing that kills them all.