Monk's Hood(Chronicles of Brother Cadfael‪)‬

Brother Cadfael is confounded when his own potion is used to kill his former love’s new husband
Gervase Bonel is a guest of Shrewsbury Abbey of Saint Peter and Saint Paul when he suddenly takes ill. Luckily, the Abbey boasts the services of the clever and kindly Brother Cadfael, a skilled herbalist. Cadfael hurries to the man's bedside, only to be confronted with two surprises: In Master Bonel's wife, the good monk recognizes Richildis, whom he loved before he took his vows—and Master Bonel has been fatally poisoned by monk's-hood oil from Cadfael's stores.
The sheriff is convinced that the murderer is Richildis' son, Edwin, who hated his stepfather. But Cadfael, guided in part by his concern for a woman to whom he was once betrothed, is certain of her son's innocence. Using his knowledge of both herbs and the human heart, Cadfael deciphers a deadly recipe for murder.