After reading all six books of this series, I'm going to voice my overall opinon for all of them here. Minor Spoilers ahead.
The world shown to us by Robbie Collins isn't without fascination. It isn't a totally bad series and has its moments. However there are two major points that were a real enjoyment-killer for me and only allow me to give a rather negative rating for this.
Firstly, the heroine from the second book onwards is way too overpowered. In this world she quickly rises from a powerful individual to a kind of messiah and we are often shown and told exactly that. She manages to jump every hurdle in her path easily. She even comes back from the dead two times and has the power to bring others back too. Much more impressive than her were the villains for me. These people who dare to confront this force of nature, that is our protagonist, rarely last more than a brief encounter with her though. They usually are defeated quickly and die slow and horribly.
Which brings me to my second negative point. The level of violance and cruellty on the side of the "good guys" is rather intense. The villains are really devilish, but the heroines aren't far behind. Some examples of what the "good guys" do would be: Torture to get information, torture to punish a villain, pitting prisoners against each other to fight to the death, killing soldiers that already surrendered, mutilating enemies as a warning, annihilating a city and every soldier, civilian and child within it, spreading illness across the enemies lands, and so on. At some (few) points the heroines lament how sad it is they are forced to commit such acts, but I never really felt that to be really the case. Most of their acts don't feel necessary at all in fact.
I found it hard to feel sympathy for the heroines. It never feels like they are in real danger anyway and all that cruelty makes them look vengeful to me. That more than anything else.
In the end I can only give two stars. Maybe you can overlook these flaws and aren't as bothered by them as I was. To me this book and the rest of the series proved to be a waste of time and money, mostly. If I hadn't already bought them all at once, I probably wouldn't have finished the series. I just hope anyone considering buying these books will at least keep the things I pointed out in mind before doing so. You can decide for yourself if it bothers you or not.