Monk's Hood: Ellis Peters

This was one of the books from the Brother Cadfael series. The story opens with a monk from the year 1138 working with monkshood oil (the poison wolfs bane). He warns others to thoroughly wash their hands as it is extremely poisonous if ingested. A man, Bonel, mysteriously dies shorty afterwards and Cadfael smells the monkshood on the dead man's lips. Cadfael then recognizes the dead man's wife as a woman he was betrothed to 40 years earlier before the church called to him. The warden believes Bonel's son murdered him because he would inherit his property. Cadfael finds out the boy doesn't know how his father was murdered and thought he was stabbed. Cadfael learns there was an older illegitimate son that actually murdered Bonel believing that he would inherit everything. Cadfael helps the murderer escape after he promises to spend the rest of his life doing good.