Death Comes As Epiphany by Sharan Newman

It's been a long time since I read this. Newman certainly knows the period and infuses the story with a sense of realism that isn't always present in fiction set in the medieval period. The mystery and accompanying romance are set against the story two of the most towering figures of the twelfth century, Abelard and Heloise, in their later years. Heloise is the abbess of the Convent of the Paraclete and Abelard is in Paris lecturing on the philosophy of reason.

Catherine LeVendeur is a novice and brilliant student at the Paraclete and Heloise sends her away "undercover" in supposed disgrace to uncover a plot to discredit the convent, Heloise herself, and Abelard. At the great abbey of St. Denis, Catherine encounters a most unusual stonecutter's apprentice. We eventually learn that Edgar is an English student of Abelard's sent on a similar mission.

Newman doesn't over-romanticize the period. It is a dangerous and uncomfortable place to be and she lets us see that clearly. We also see superstition driving people to madness. The motives behind the crimes are characteristic of the period, not the sort of thing that one would expect in a mystery of a later period. On the other hand, the basic motivations are still greed and revenge, but the form they take is not much like what we find in novels of the twentieth and twenty-first century.