There are many ways to write a bad historical novel, but perhaps the most effective one is to be careless with the details. And this novel put me off from the first page; when someone asks Margaretha Zelle (Mata Hari) how old she is and she answers "twenty-five". Now, this first chapter is supposed to be taking place in 1904 and Zelle was born in 1876. She was 28, not 25.
Two pages later, Mata Hari says "I learned some Hindi." That's no good, again. Margaretha Zelle is not reported having ever lived in a place where people spoke Hindi. She lived in Malang, a city in the region we call today Indonesia, in the island of Java. And they speak Javanese, there, a completely different language.
Later on, there is mention of Mata Hari dancing with a snake, something that was never reported by her biographers. We cannot say it is untrue, but it surely rings untrue.
I am sure there are more mistakes like these in the novel, but I am not going to check every page. After the first 10 pages, I just skimmed through the novel to see what it was about. And whatever it was about, it was nothing interesting. I don't know what Ms. Moran had in mind with writing this story, but the Mata Hari she describes just doesn't ring true. The novel never lifts off. For me, it was just boring.
This is a general problem in writing historical novels. It is an exceedingly difficult task and only a few writers succeed. You have to know all the details of the historical period, not so much because your novel will be read by historians, but because consistency is everything in a novel. And in a historical novel, even the casual reader is perfectly able to detect inconsistencies when they tend to accumulate because the author didn't do his/her homework.
Then, the difficulty is compounded if the author tries to tell the novel in the first person from the mouth of a known historical character. That's almost impossible, although some exceptionally gifted writers managed to succeed. An example is Marguerite Yourcenar's "Memoirs of Hadrian". But Ms. Moran's memories of Mata Hari, well, very sorry, no.