The Leper of St. Giles
by Ellis Peters. This is the fifth of the Brother Cadfael mystery series and so far the best one I've read.
In this story, a rich Norman aristocrat is murdered on the eve of his wedding to a woman young enough to be his grand-daughter. Suspicion points to a young suitor to the girl, and Brother Cadfael must get to the bottom of a fairly tangled affair.