Hemlock & Silver By T. Kingfisher

Healer Anja knows little of politics but much of poisons. When she is summoned to treat the mysterious illness afflicting the king’s daughter, she finds herself against the clock, desperate to track down the source of the poison killing Princess Snow. But the chance discovery of a strange alternate world inside a magic mirror leads Anja to darker discoveries, including what really happened to Snow’s dead sister, Rose, and why their mother seemingly went mad and cut out her heart.

Aided by a taciturn bodyguard, a narcissistic cat, and a late Renaissance understanding of the scientific method, Anja must navigate the mysteries of the mirror world before the dark queen that dwells within rises to threaten them all