Dead Man's Ransom (Cadfael #9)

Brother Cadfael uses his knowledge to pick at clues until the killer can be found.
A character driven story, it doesn't go in for convolutions that some mystery authors chose to show off how clever they were, and as such has much more re-reading value.
The period it's set in acts as a handy backdrop for the events that take place, and Peters has a good way with people's language and the way they talk.