The Goldsmith's Daughter by Kate Sedley

King Edward IV trembles as he decides the fate of his sibling. And Richard, Duke of Gloucester, plots, trying to find a way to save George from being put to death by their eldest -- and powerful -- brother, the King. So when the Duke sees his old and loyal servant, monk-turned-travelling salesman Roger the Chapman, among the crowd at the trial he recognises that he has a chance.

If only the chapman-sleuth could prove that the kinswoman of the King's favorite leman hadn't poisoned her taciturn husband. If Isolda Bonifant, the daughter of a well-established London goldsmith, were innocent and her name cleared, then Edward's chief mistress -- cousin of the accused Isolda -- would be more than willing to do the wily Duke's bidding. But Roger the Chapman must act fast and, in a complex case like this one and with the pressure of Richard of Gloucester upon him, he can't simply rely on his intuition.