"The Cannon Law" is the direct sequel to "1634: The Galileo Affair". It continues the story of the Frank Stone and the Venetian CoC, the story of Sharon Nicols and Ruy Sanchez Casador y Ortiz, and adds the youngest Cardinal Barbarini. Also how all of these are impacted by the mad plots of Cardinal Borja.
Of the novels in Eric Flint's alternative history this might be one of the easiest ones to start with. There are only a handful of important characters, the plot is tightly focused, and all the information the reader has to know is included in short asides.
I enjoyed this story as much for the action as for the outrageous character of Ruy.