The Water Devil (Judith Merkle Riley, 2007) is book three of a trilogy featuring Margaret of Ashbury and her life of adventure and mysticism in 14th century England. You really want to begin at the beginning with this series--the first book is A Vision of Light and the second In Pursuit of the Green Lion. Otherwise you'd have no idea who was who in The Water Devil nor why you should care about them.
And you should read these books. They're just plain wonderful, with strong writing and memorable characters, especially strong-willed, mystical yet practical Margaret. I feel like Riley does a better job than most authors at capturing the feel of the Middle Ages--the energy, the social tensions, the color.