I've just read a synopsis of the book (I've never paid £100 for a book, and I'm not about to start,) and my reaction is "so what?"
Russia had allied itself to Germany, and taken the opportunity to grab half of Poland for itself. Britain was hardly likely to view Stalin as a potential ally, in fact there was a distinct possibility that he might assist Germany militarily (I doubt that Britain knew about the clause in the Germany/Russia pact that, in the event of one going to war, the other would remain neutral.)
Any nation that doesn't prepare for any eventuality is doomed to play catch-up, with all that entails; the seriousness with which the plan was viewed can possibly be judged by the fact that, during a large part of 1940, Slessor was part of the Purchasing Commission in the U.S., hardly the ideal place to plan for an attack on Russia.
Much is made of Stalin not trusting the British, but he didn't trust anyone, even Russians, which is why he murdered so many of his own countrymen. All that mistrust did was cause thousands of his soldiers to die (not that he cared.)
The twin-engined aircraft, used for the photos, would have been either a Blenheim, or Cotton's Lockheed 12A, which were painted in Cotton's "Camotint" invention, a green (not blue) paint. It would not have been used after September, 1940, since it suffered bomb damage at that time.
There were certainly plans to use the Lockheed in the Middle East, because Cotton had two extra 70 gallon fuel tanks installed, which increased the still-air range to 1,600 miles, and this was done in June 1939.