Series: Brother Cadfael

I sometimes call these CSI: Shrewsbury. At least the BBC TV shows. They're rather long-ago historicals (about 1135 to 1145), about a lay brother at the monastery of St. Peter and St. Paul in Shrewsbury. He seems to have spent his life as a soldier, including a lot of time out of the country on crusade, and then come back to retire to the monastery. He's their herbalist and in charge of their herb garden, and he also frequently gets called upon to deal with some of the nastier interactions with the secular world.

He fairly often uses clues from bits of plant matter found on victims or at crime scenes, combined with his detailed knowledge of what grows in the neighborhood, as major clues to solve crimes.