Sister Frevisse is the niece of Chaucer and this relationship inspires the titles of the books of the series.  It also gives her an entrance into the worldly sphere of those who are well-to-do or well-connected politically.  She is a rather difficult character to work with - although she has a genuine vocation and entered the convent of her own will, she is also in frequent rebellion against the constraints of her life.  

Some of the mysteries are set in the wider world and some in Frevisse's convent.  In either setting, the period detail and research are impeccable.  Margaret Frazer is not one of those authors who constantly point out how badly things smelled and how miserable life was, but neither is she inclined to prettify or romanticize the Middle Ages.  The longevity of this series attests to how well-written and how interesting these mysteries are.