Hemlock & Silver by T.Kingfisher

Arsenic, white thorn apple, wolfsbane...

Healer Anja regularly drinks poison.

Not to die, though, 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. 

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...