As with many books or stories that we get involve with, the characters and their relationships to others in the environment is an important as the mystery. Ellis Peters (Edith Pargeter) strikes a balance between the characters, history and the mystery. Sprinkled throughout is faith, and a chance that they (the monks) may be correct in the explanation of saints and how the world works.
The external environment is the ongoing 11th century civil war between English King Stephen and his sister the Empress Maude. We also have references to the different societies as they travel to Wales. These become more relevant as the series progresses.
The inward struggle between faith and power is depicted as an individual monk is persuaded or wants to be persuaded to go on a mission to retrieve a neglected saint "St. Winifred." She lies in Wales and it happens that Brother Cadfael has a Welsh background, so he is charged with supporting the mission.
If you saw the movie you will immediately see the differences between it and he book. One main point is the fact that the monk was cured before the trip. The best difference is reviled with the detection and solution to the mystery.One Corpse Too Many: The Second Chronicle of Brother Cadfael