You can see the outlines of a coherent hypothesis in “How America Lost Its Secrets.” Perhaps Snowden was planted at the N.S.A. by either Russia or China, or by both. Perhaps while he was there he worked with other, as yet undetected, insiders who were also serving foreign powers. Perhaps in Hong Kong he put himself into the care of Chinese handlers who debriefed him extensively during the nearly two weeks between his arrival and his self-outing. Perhaps the same thing happened in Moscow during the first 37 days after he landed there, when he seems to have been hiding somewhere inside the airport security perimeter. Perhaps his reward for, in effect, defecting has been the odd protected life in Russia that celebrated spies like Kim Philby and Guy Burgess previously enjoyed. Perhaps his media-abetted role as a whistle-blower was merely a counterintuitive (because it was so public) new form of cover.

Epstein proves none of this. “How America Lost Its Secrets” is an impressively fluffy and golden-brown wobbly soufflé of speculation, full of anonymous sourcing and suppositional language like “it seems plausible to believe” or “it doesn’t take a great stretch of the imagination to conclude.” Epstein’s first book, “Inquest,” published more than 50 years ago, featured another mysterious young man who spent time in Moscow, Lee Harvey Oswald. This book has a greatest-hits feeling, because it touches on several of Epstein’s long-running preoccupations: Russia; the movie and media businesses; the gullibility of liberals; and, especially, the world of penetration, exfiltration, false flags and other aspects of counterintelligence. The spirit of James Jesus Angleton, the C.I.A.’s mole-obsessed counterintelligence chief during the peak years of the Cold War and evidently a mentor to Epstein (he’s mentioned several times), hovers over these pages.

Sometimes it seems as if Epstein so much enjoys exploring the twists and turns in Snowden’s story — his encounter with Snowden’s mysterious lawyer in Moscow, Anatoly Kucherena, is especially memorable — that he doesn’t have an overwhelming need to settle the questions he raises. The sentence from The Wall Street Journal quoted above appears almost verbatim in the book, but it’s immediately followed by this: “These severe accusations generated much heat but little light. They were not accompanied by any evidence showing that Snowden had acted in concert with any foreign power in stealing the files or, for that matter, that he was not acting out of his own personal convictions, no matter how misguided they might have been.” But then Epstein spends many more pages considering, and not dismissing, the very same severe accusations, and ends by saying that “Snowden’s theft of state secrets . . . had evolved, deliberately or not, but necessarily, into a mission of disclosing key national secrets to a foreign power.”

Image Edward Snowden makes the case for a presidential pardon via video link at a news conference, September 2016. Credit... Spencer Platt/Getty Images

This is Epstein’s primary conclusion: Even if the American public was a partial beneficiary of Snowden’s revelations, the main beneficiary was Russia, which to his mind couldn’t possibly have failed to take possession of all the material Snowden took from the N.S.A. Whatever caveats he uses and whatever hard evidence he hasn’t found, Epstein clearly wants to leave readers with the impression that Snowden remains in Russia as a result of a deal exchanging his information for its protection. He repeatedly hints that he has reason to be more certain about his conclusion than he’s able to say in print.