This is book 4 of the series The Mistress of the Art of Death.
Adelia Aquilar is from Sicily but forced to remain in England by King Henry II. She is a 12th century anatomist who was requisitioned by Henry Plantagenet to solve a series of murders. Once the case was solved, Henry decided Adelia was much too useful to him to allow her to return to Italy so she has been forced to build a life in England.
Now, Henry is requiring that she accompany his daughter, Joanna, as she travels to Sicily to marry William II King of Sicily. Adelia is thrilled to be returning home but Henry is holding her daughter hostage to ensure her return. Adelia must also help Henry keep a mythical treasure from his scheming sons while a madman stalks her, determined to end her life as she ended the life of his lover over two years ago.
I really enjoy these books. Franklin seems to do a great deal of research regarding the politics of the day but also where medicine was at the time. I'm wondering how many more books are to come since she has come to a point in history when the Church really took over medicine and everything became much more conservative in both England (especially after the death of Henry II) and Italy.