The Novice's Tale by Margaret Frazer
It is the year 1431, and the nuns of St. Frideswide live quiet lives. But with the unwelcome visit of the hard- drinking, blaspheming Lady Ermentrude their lives are suddenly not so quiet. But just as suddenly the Lady is found dead. Sleuth Sister Frevisse gets on the case a detects a cleaver web spun to entangle the innocent.