The Leper of St. Giles by Ellis Peters, 199 pages

October 1139. A savage murder interrupts an ill-fated marriage set to take place at Brother Cadfael's abbey, leaving the monk with a terrible mystery to solve. The key to the killing is hidden among the inhabitants of the Saint Giles leper colony, and Brother Cadfael must ferret out a sickness not of the body, but of a twisted mind.