One Corpse Too Many
by Ellis Peters. This is the second in a series of mystery books about Brother Cadfael, a worldly Benedictine monk, set in 12th Century England.
This story is set during the civil war following the death of Henry I (son of William the Conqueror). Henry's heir was his daughter Maud (or Matilta in some history books) but her cousin Steven assumed the throne since she was in Normandy at the time. Steven won the civil war, but Maud's son, Henry II, succeeded him.
Shrewsbury and the abbey are caught up in a siege by Steven's army. Steven is victorious and Brother Cadfael is assigned the duty of burying the dead soldiers. Among th dead he finds evidence of a murder victim, tossed in among the dead soldiers so as to go un-noticed. He unravels the mystery and makes and important friend in the process.
This was a very readable book and entertaining. I think it's best to read these books in order, as one story refers back briefly to the one before.