by Ellis Peters. This is the third in a series of mystery books about Brother Cadfael, a worldly Benedictine monk, set in 12th Century England.
In this story, Brother Cadfael is called upon to solve the murder of a wealthy Englishman visiting the abbey with his wife. There are several twists in this story that give insight into Cadfael's past. Also, there is a healthy dose of Welsh-English politics and contrast of common law. Hugh Beringer, who the reader meets in the previous book, and appears to be a regular character going forward.
"Monk's hood" is also know as "wolfsbane," and (as you learn in the book) was used as an ointment for sore joints. But it is a deadly poison if taken by mouth. So that gives you a clue out the crime that Cadfael solves in this book.