When I first picked up this book, I quickly realised that I had no real idea what had happened in the majority of this series. I followed the main plot, but the plot revolving around the ark and the city minds? No clue. In the previous books it had been possible to dismiss as a subplot, but I quickly realised that sometime between the start of the second book and the start of this one, it had actually become the main plot. Thankfully, the author included a summary of the story so far, so I wasn't too far behind!
Once I actually started getting into it, I found this book a lot easier to follow than the previous ones. The threat was more present than ever before, and I started to feel like I actually understood the world and the mechanics behind it. I liked getting to revisit these characters, especially Thurin and Erris, and although I'd hardly call him likable, I also really enjoyed seeing Theus again. 
This book, and the series as a whole, is one I think I'd need a reread to fully understand, and I also highly recommend reading them in close succession, now that they're all released. I read these with a long gap between each one, and that definitely didn't help me in following the overarching plot. However. it is still a pretty good series, and one I think is worth reading.