Hemlock & Silver, Paperback/T. Kingfisher

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 Anjas 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. Nothing, 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