Hemlock & Silver by T. Kingfisher
Anja drinks poison. A lot.
Healer Anja, the usual competent Kingfisher protagonist, is hired to uncover the mystery of how someone is being poisoned. Cats, bodyguards, some really creepy bits, and a satisfyingly true to life resolution of the final, niggling “plot hole” make this an enjoyable twist on a certain fairytale.