Dead Man’s Ransom - Ellis Peters

The year is 1141 and civil war continues to rage. In the battle of Lincoln the sheriff of Shropshire is captured and the king himself taken prisoner by his enemies. Nothing more natural than that an exchange of prisoners should take place: the sheriff to be recovered in place of a young Welsh lordling seized in the course of a misguided assault on a convent. But before the exchange can be completed, one captive is murdered. To Brother Cadfael, who first notices the evidence of unnatural death, falls the task of gathering enough clues to prove it.