The author strongly hints at who these contacts were, but ultimately declines to identify them. In the book credits, he makes the excuse that he was unable to get their permission.
You know permission isn't necessary if you are disclosing facts, so I have to wonder if he feels a moral obligation to keep silent for some reason such as, perhaps, they helped him get out of Russia shortly after the Soviet Union collapsed and resettle in the United States.
What's your theory?
A subsequent former KGB author who penned his book about 10 years later has a theory posted at the link. I have not read his book so am unable to comment on the veracity.
The only thing I can say is that if you made a list of journalists based in Washington with pro-Soviet sympathies and those without, the former list would be much longer and, thus, would be a good place for a KGB contact to hide.
It sounds like a good read. The cold war era shouldn't be forgotten. We're now paying for having won the war but, IMO, losing the peace.