There are so many theories floating around about what really drove Henri III of France. Was he really gay? Was he as incompetent as later historians claimed? Did he really torture his sister and her friends as much as I and other writers have portrayed?
Martha Walker Freer wrote exhaustive biographies of French monarchs and their Queens. While the books are packed with detail, some of those details are a bit salacious. You can mine some great stories from them, but not everything she wrote was based on truth. You can download her three volume series about Henri III from Archve.org.
I used the third volume as a starting point for retelling the Duc de Mayenne’s siege of Tours. She goes into so much detail that I tried to find what sounded credible and then find other sources to back up what she wrote. She claims that her books are based on exhaustive research of original documents at the French National Archives, but I think some of her conclusions lean too much on the side of melodrama.
If you’ve got plenty of money to throw around, Robert J. Knecht is a foremost expert on Sixteenth Century France. He focused heavily on Francis I, so it’s hard to find a lot of good detail in his work on Francis’ descendants. I have two of his books, and the sections on Henry III are literally the same thing, copy and pasted.