There is some interesting material, but the whole feels a bit shallow. For instance, Earley doesn't seem to know much about the European Union and his account of COREU telegrams is confused and inaccurate; when his details on points that I know about are poor, I naturally become suspicious about the rest. Though other bits rang true: there is one beautiful Kafka moment (which Earley calls 'a catch-22 situation'): one section of the Russian intelligence establishment in New York had the job of recruiting and converting FBI and CIA agents, but were also forbidden by their own regulations from having any contact with known FBI or CIA agents. This of course led to significant padding of reports, making it appear that the rather few genuine US contacts were more impressive than they were.
Earley does not examine the extent to which Tretyakov's work actually affected Russian policy and actions more than would have been the case had he been an ordinary diplomat, and that is the biggest gap in the book (though of course it's a much broader question equally applicable to Western intelligence agencies). There are two interesting passages about spreading disinformation among the academic community. I remember the Transdniestrian astroturf affair from a couple of years back, as chronicled by Edward Lucas at the time, which was a rather good example of this; but a more audacious claim is that the KGB simply invented the idea that the widespread use of atomic weapons would result in a "nuclear winter" in order to strengthen the anti-nuclear lobby in the West. I've no idea what the current status of nuclear winter theory is among climate scientists, and Earley doesn't investigate this, simply accepting Tretyakov's account that his colleagues made it all up, and again I wish he had checked a bit further.
One of the more interesting but less believable claims in the book is that Strobe Talbott, then a senior US official, was 'played' by a Russian official who was really in intelligence but pretended to be matey with him. Talbott, asked to respond, contends (entirely credibly, though Earley doesn't seem to believe him) that he always expected and believed that his interlocutor was passing the entire contents of their conversations back to various contacts in Moscow, and spoke to him on that basis; he doesn't add, but might have, that that is what makes such conversations worth while in the first place. The fact that Tretyakov (or his FBI/CIA handlers) wanted this story published is itself perhaps significant.
There is a cautionary tale there. My own policy with contacts who I know or suspect to be in that line of work is to treat them as I do 'ordinary' officials, or indeed reasonably motivated graduate students. If my interlocutors fancy they are getting better information from me than their competition, that is their lookout; any such conversation, from my point of view, is always at least partly about influencing government decision-makers or the wider epistemic community. This is a game played in both directions, of course; the details of how one of the world's most famous services handles HUMINT are fascinating, and the general guidelines and specific judgement calls that Tretyakov and his colleagues made when deciding how and when to develop contacts make for interesting reading.
The other interesting human story, though of course one has to treat it with due caution, is the slow disillusionment of Tretyakov and his family with Russia after the fall of Communism: the increasing surrender of Russian territory as well as the economy to criminal oligarchs, backed by what passed for the central government, must have been an awful process of disillusionment for all patriotic Russians. Each has made their own accommodation with the new state of affairs; Tretyakov chose to turn his back on it and seek a new beginning. He enjoyed it for less than ten years.