Don't read this book; it is really bad.
It's a stylistic disaster; it is divided into three parts, the first of which is a confused mish-mash of inter-related "fairy-tales". It mixes up Russian history with ideas from Arthur Ransome's book about - Russian fairy-tales! This, I think, is supposed to be clever, because the book is a fictionalised tale of Arthur Ransome's time living in Russia, before, during and after the revolution that brought Lenin to power. In fact it's just an incoherent mess. The second part is a straight-forward third-person narrative, describing Ransome's first stay in Russia. It's a vast improvement on the first part, but nothing special. The third part switches to the first-person (!) to cover Ransome's second visit to Russia. It's completely jarring, as if the switch from "fairy-tale" to 3rd person wasn't enough of that sort of thing - at least the former transition was an improvement - this is a deterioration.
The conceit is that Ransome, well-known children's author, was a spy; this is based on documentary evidence declassified shortly before publication of the book. Some of this is reproduced in an appendix and is by far the best part of the book. Was Ransome a Bolshevik agent? Some people in Britain's Intelligence community thought so. Others were convinced of his trustworthiness - and Ransome had an official designation as a British agent. Also, MI5 and MI6 are pretty famous, but have you heard of (the now defunct) MI1c?
Here's the second reason why the book is really bad: the above conceit, Ransome's love-affair with Trotsky's secretary, Russian winter, revolution, civil war, WW1 and Sedgwick makes the book somehow drab and undramatic! How is that even possible!?
The thing that gets me down about all this is that Sedgwick can write really well - The Book of Dead Days and Kiss of Death are powerfully atmospheric and compelling. The Raven Mysteries are tremendous fun and GothicK with a capital K (and G)(but you can see that). I can strongly recommend those books. But really, don't read this one.
Sidenote: Sedgwick thinks Napoleon's defeat in the winter of 1812 was merely an accident of the weather: clueless!