Ellis Peters: Monk's Hood

From the Publisher:
Gervase Bonel, with his wife and servants, is a guest of the Shrewsbury Abbey of St. Peter and St. Paul when he is suddenly taken 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 by two very different surprises. .

In Master Bonel's wife, the good monk recognizes Richildis, whom he loved many years ago before he took his vows. And master Bonel himself has been fatally poisoned by a dose of deadly monk's hood oil from Cadfael's laboratory. .

The sheriff is convinced that the murderer is Richildis' son, Edwin, who had reasons aplenty to hate his stepfather. But Cadfael, guided in part by his tender 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. .

Ellis Peters: Monk's Hood. The Third Chronicle of Brother Cadfael. Warner Books, ISBN: 0446403008 (November, 1992), $6.99.