Through the Valley of Death by E.M.A. Allison

Brother Anselm is dead, and it's not the plague-it's murder. Who would want to kill a monk in a Cistercian monastery in 1379? As it turns out, this brother has a lot of enemies. The unhappy task of finding the killer falls to Brother Barnabas, a monk who's gifted in logic, but dangerously innocent of the ways of the world. Before his investigation is over, Barnabas learns more than he really wanted to know about the evil that men-and even monks-do.