Ellis Peters is in a class by herself. Before her death at the age of 82 in 1995, she wrote a series of 20 mysteries set in England and Wales during the 12th Century. (She also wrote many other books under her real name, Edith Pargeter.) In the Cadfael mysteries, her "detective" is a former Crusader who has decided to spend his last years as a monk in a monastery in Shrewsbury. I have found the Brother Cadfael in the books to be far more interesting than the Brother Cadfael of the television series. Many of the books in the Cadfael series are as much (or more) historical romances as they are mysteries. Don't start any book in this series unless you enjoy a strong dose of romance and medieval English history mixed with mystery. The Cadfael series (like most good series) is more enjoyable if the books are read in sequence. Dead Man's Ransom is the ninth book in the series.
In Dead Man's Ransom, Brother Cadfael is called upon to solve the murder of the Sheriff of Shropshire, Gilbert Prescote. Suspicion falls on a Welsh captive and on others who have a grudge against the stern sheriff. Cadfael's investigation is hampered by Welsh raids along the border and by continued strife within England.