Prompted by the question on historical fiction...
I've just finished reading "The Devil's Queen," a historical fiction about Catherine de Medici. In it, the Duke d'Anjou presents Catherine de Medici with a letter he says is from Henry of Navarre to a commander in the field, ordering that an army attack Paris on August 25, that Catherine & Henry be killed, and that Charles be taken prisoner until he abdicates. He claims he has spies who've seen the army amassing. On this evidence, Catherine determines that the royals/Catholics must strike first and attack Huguenot leadership on the 24th, Bartholomew's Day. Together, she, Henry, and Gaspard de Saulx get Charles to sign off on the assassinations.
In the book, the Duke was a forger and a liar. He created a fake Huguenot plot to spur the massacre to ensure that when his sickly brother died the Huguenots would be too weak to represent a threat to his reign.
A quick look online says there are several theories about exactly how the massacre came to be, and that "the Huguenots were plotting against us" was an excuse given at the time. Is there any evidence, though, that Henry fabricated the Huguenot plot?
historical fiction
.... and fiction it is. I am not aware of any credible historical mention of the Duke of Anjou presenting a faked Huguenot plot.
That there were 4000 Huguenot soldiers outside of Paris under the command of Coligny's brother-in-law Teligny was well-known and not a secret at all. After all, there were a great many Huguenot leaders and their families visiting Paris to attend the wedding of Henry III Navarre and Margaret. Teligny himself spent time negotiating with the king Charles IX, and played a few games of tennis with the king. The expectation was that the Huguenots would stay for a while in order to negotiate peace.
Most popular documents citing the presence of 4,000 Huguenot troops trace back to Holt's book on the subject, in which the presence of troops was presented flatly not as a surprise to anybody. There was no mention of finding an attack order written by Henry Navarre.
Further, Henry Navarre himself was not murdered in the massacre, on account of his marriage to Margaret and his abjuration of Calvinism to rejoin the Catholic Church. The Catholic fear preceding this was that he king Charles IX had fallen under sway of the Protestants.
At any rate, historical fiction or not, you have to distinguish the stages of the massacre.