I've just finished reading his "An Officer and a Spy". Excellent, I can't recommend it enough.
It's based on the true story of the infamous "Dreyfuss affair" in France at the end of the 19th century. All the characters and basic facts of the story are true, but Harris has fleshed them out with dramatic licence to make a highly readable historical novel.
Dreyfuss, a jewish French army officer, was accused of passing military secrets to the Germans. Tried and found guilty, he was cashiered and sent to solitary confinement on Devils Island. The head of French military intelligence, Georges Picquart, who at first helped convict him, gradually became convinced that they had got the wrong man, and the trial was a stitch up by the most senior army officers and politicians.
The novel is the story of how he overcame impossible odds against dark forces, despite being ostracised, kicked out of the army and threatened, to prove Dreyfuss innocent and get both of them re-instated. Top class writing.
Roman Polanski made a french film of the novel in 2019 which was also highly rated. I'm currently looking for it.