I'm giving it a 3/5 for the actual real-life story, NOT for the way it is written.
It's a pretty unbelievable true story of a working-class Australian girl, who runs away at sixteen, gets a job, travels to Europe, gets a qualification in London, moves to Paris, parties in Paris whilst working, meets a French guy, gets married. Meanwhile, WWII breaks out - and Nancy, the protagonist, has an avowed hatred of Nazis (understandably). So she vows to do everything she can to fight against them, and her life completely flips upside down. She works in the Resistance, eventually flees France to survive, then parachutes back in with the support of the Allies and leads large bands of French rebels (the Maquis) to stage attacks on the Nazis in France - contributing to the success of eventually kicking the Germans out of France again by the end of the war.
So, Pros: Incredible story of achievements, very impressive, especially in terms of female representation.
Cons: I didn't really connect with Nancy the character at all. I do not feel like we would have gotten along, for various reasons, unless we were both somehow engaged in battle against some Nazis. In addition, I really disliked the way the book was written...
A) There's already two books about her life - including an autobiography - so why, exactly, write a THIRD book?
B) The author adds in all these over-the-top Australian references and Australian slang - but honestly, I'm unfamiliar with most of it (and I'm at minimum 5th-generation Australian). And it's usually not particularly relevant to what is being described, and I felt that it detracted from the momentum of the story. It just seemed almost a little patronising. There's no need to put in silly Australianisms everywhere - it doesn't make it any more authentic. It just makes it frustrating to read.
C) The author provides little to no critique of any of Nancy's decisions, actions or character traits. Of course, a biographer is unlikely to heavily critique the person he is writing the book about, but she's not a goddess, mate (this, for example, is an appropriate place to insert Australian slang, because it is an informal online review, not a professional biography). Was she a very adventurous, determined war heroine? Yes. Was every decision she ever made 100% excellent? No. There's really very little moral questioning ever shown, beyond sayings like "Well, Nancy wouldn't stand for that!" or stating how much she values freedom.
D) There's this underlying attitude that one of the things that made Nancy special was that she could be "one of the men"; e.g. be extremely accomplished at "manly" pursuits such as binge drinking and swearing. Somehow, this is supposed to show how legendary Nancy was. However, I would suggest that it is a sad indictment of the underlying patriarchal mentality of our society, that in order for a woman to be considered great, she must be considered "one of the men".
I could go on further, but they were the chief concerns I had with the book. If you like war stories and don't mind stories being written in an odd way, then go ahead and read it. But if not, then probably don't. I don't mean to disparage Nancy's actual story - the essence and action of which I think is well represented within this book.