An intelligently crafted problem, superior depiction of the historical period, and vivid, diverse characters make this a mystery that transcends the genre.
This is the only Cadfael book that I've read (so far), and so my observations are not influenced by other books in the series. From the start, I was drawn in by the convincing evocation of medieval Shrewsbury, a little universe consisting of castle, town, and monastery, and its population of knights, monks, bondsmen, ladies, and Flemish mercenaries. Cadfael, the middle-aged warrior-turned-monastic, with his spiritual outlook and worldly knowledge, is a strong and lovable protagonist. The wily, nonchalant knight Hugh Beringar makes a worthy adversary.
The mystery itself--the "one corpse too many" found among the pile of executed enemies of the king--is actually secondary to the main story of the book: whether Cadfael will succeed in helping a young fugitive, Godric, escape the wrath of the king. The skillful interweaving of these plots, along with not one but two nice little love stories, make this book a refreshing change from the standard mystery. The juxtaposition of spiritual and worldly values is well handled and gives the book a feeling of depth.
Most novels nowadays are too poorly written to be worth finishing. Not this one: Peters's prose style is vivid and clean, comparable to Mary Stewart when she was at the top of her game with her Merlin books. I give it 4 stars out of 5 only because you need somewhere further to go for the truly great works of literature. As far as mysteries go, they don't get better than this.