Priscilla Royal's series of Medieval Mysteries is not for every mystery reader. You have to have both a good knowledge of the period, as well as an abiding interest in it. But if you're an amateur "medievalist", you'll enjoy Royal's writing.
This book, "A Killing Season" is the eighth in the Prioress Eleanor of Tyndale series. Eleanor is a highborn woman who becomes leader of the Order of Fontvraud in 13th century England. She serves over both men and women religious. Tyndale Abbey is the setting for most of Royal's stories and the family of nuns, priests, brothers, and lay officials keep Eleanor and her aid, Brother Thomas, busy with both the clerical and worldly problems. (Brother Thomas is himself an interesting character. Charged with sodomy, he was sent to Tyndale Abbey for both penance and to serve the court of English king). There's a lot of intrigue on all levels in Royal's writing.
"Season" is set in a castle on an island in the freezing north of England. Eleanor, Brother Thomas, and several "healers" are summoned there by Eleanor's brother, Sir Hugh, to wait upon a comrade of Hugh's, Baron Herbert, who has returned home from the Holy Land (Outremer) and is having trouble adjusting to life on the home front. Herbert had left home years before as the father of five young sons, and has returned to find his sons dying at a rapid rate. Three are gone already - due to "accidents" - and the younger two are threatened. Herbert cannot - literally - face his family and he's gone into hiding within the castle. Eleanor, Hugh, Thomas, and several healers have to ferret out secrets to save lives.
"A Killing Season" is not Royal's best book. It's slightly draggy and lacks the "oomph" of her previous books. I can heartily recommend it to readers of the previous books in the series, but for those who haven't read her books, I'd say, start with the earlier ones.