This is book 3 of the "Brother Cadfael" series.

This note contains spoilers for the book.

A man is murdered, with a medicine Cadfael had prepared. He, and the abbot, feel it's necessary to find the killer to maintain the honor of the abbey.

None of it would have happened if the abbot hadn't been called away to account for his stewardship -- and probably be replaced. A charter is put in limbo which would disinherit some people and benefit the abbey.