I finished the book yesterday. As others have said, it was a light and fun read (it took me two days). The plot, while basic, moves quickly and features strong action, akin to an episode of New Who. I don’t think it fits into Season 1 very well, though, because this is a much more traditional adventure than anything we’ve seen this season. That’s not Cook’s fault, though, because how would she know what the next season was like when she wrote the book before the episodes were out?
Ruby is written very well, and I could imagine Millie saying and doing those things. The Doctor felt a lot like a generic New Who Doctor to me (a mix of 10/14, 11 and 15), and I didn’t get the Ncuti vibes from him (perhaps it was the lack of crying or the fact that he actively took charge of most situations rather than simply freezing and letting others do the work). The other characters are equally compelling and engaging. Ran serves as a valuable ally, exemplifying a character who chooses to direct her own life instead of allowing external influences to shape it. Her sisters are also fascinating, and the relationship between them is the most captivating aspect of the plot.
Genetrix is an intriguing villain. I liked her motivation and MO, and I thought she was effective enough. The final confrontation between her and the Doctor in the TARDIS is nice. The use of squidlike parasites to possess people made me think of Last Christmas or Alien, and for some reason I imagined Genetrix to look like Ursula from The Little Mermaid 
I was slightly disappointed that the setting and time period didn’t come into play more. A wintry Russia of 1242 is a unique setting, and I expected us to spend more time with the human characters (the Prince, for instance), learning about the era and The Battle on the Ice (not a battle I was familiar with, so I spent a lot of time reading up on it after finishing the book! DW is educational, babies!) We primarily spend time with the Doctor, Ruby, and the Rentarans, which led me to believe that the setting and period of the story were underutilized. Cook could have set this story on any snowy alien planet or anywhere else in Earth’s history. The setting and zombies also reminded me of The Deviant Strain, but this one was a much better book, thankfully.
Georgia Cook writes good action and has a good grasp of modern Doctor Who stories, so this one is a delightful read. Her dialogue flows, and she gets the humour and character beats right as well. For me, it’s just some of the characterization, the familiar plot, and the wasted setting that slightly drag this down.
6.7/10