The Merchant of Death — C.L. Grace

This third novel featuring medieval physician and chemist Kathryn Swinbrooke opens just before Christmas in 1471. Snowstorms have blanketed the city of Canterbury. Kathryn and her cook Thomasina are busily preparing for the holiday, when terrible news arrives: The painter Richard Blunt has confessed to killing his young wife, along with two men who were dallying with her.
Kathryn is disturbed by Blunt's serene demeanor, but before she can articulate her suspicions, another death captures her attention. A tax collector, Sir Reginald Erpingham, has been found dead in his room at the Wicker Man tavern, and the King's monies have been stolen. Kathryn quickly determines that the collector was murdered, perhaps by poison, and begins questioning the guests at the tavern.
Meanwhile, there are patients to be cared for, a practice to build, and a household to maintain - but Kathryn must put aside these pleasant duties if she is to find the link between Richard Blunt and the strange events at the Wicker Man tavern.