This is one of the Sister Fidelma series which is a terrific series! This is one of the earlier books before Fidelma is in a relationship with Brother Eadulf. She is a 7th century nun in Ireland who is also trained in a position that's similar to an investigator.
Fidelma is sent to a female abbey where a headless woman is found in their well.
One of Tremayne's more convoluted mysteries, this one finds Fidelma called to investigate a decapitated body found in a remote abbey. On the way, the ship she is on discovers an abandoned Gaulish merchant ship with no crew or cargo, and Fidelma finds on board the missal she gave as a present to her friend Brother Eadulf. As Fidelma investigates, another decapitated body is found in the abbey. As she follows the trail of clues it becomes apparent she might herself become a target before she untangles the knots in this clever plot of greed and intrigue. Another excellent addition to the series.