Told in alternating chapters switching between the voices of WWI-era Eve and post-WWII Charlie, I'd give this one 3.75 stars if that were possible -- somewhere between "good" and "very good," with the "very good" being Eve's story.
To wit: Eve's chapters are evocative and effective -- at their best, fast-paced and tense, as any good spy story should be. They are also quite grim; the personal toll endured by Eve and her fellow spies is bleak, bleak, bleak, and the tone of the book (at least in Eve's chapters) was darker than I anticipated. That said, I had never before heard of Lousie de Bettignies or the Alice Network, nor given much thought at all to the roll of female spies during the World Wars, and for those reasons alone I think this book is a success in that it sheds light onto the extraordinary sacrifices that were made by those women. Eve is a compelling character, both fierce and formidable, and her journey from innocence to what you might call emotional ruin is intense and affecting. This novel makes clear that the horrors of war are not reserved for the battlefields alone, and the message that heroism can take many forms is an important one.
Now, the not-so-good:
I think that Charlie's chapters are almost entirely unnecessary, much weaker than Eve's, and that the book would have been much stronger and tighter without the dual perspective. As some other reviewers have said, I found myself hurrying through Charlie's comparatively dull chapters to get back to the real action with Eve. The character of Finn in particular seems extraneous (and borderline cliched) to me; the real star here is Eve and her journey and the rest is often a distraction from that.
My verdict: uneven, but worth the read for the Eve sections.