What was Edward Snowden’s real motive in stealing hundreds of thousands of classified documents from the National Security Agency? Journalist Edward Jay Epstein believes he has unlocked a sensational answer: that even if he began with the intention of exposing official wrongdoing, by the time he arrived in Moscow he was an active “espionage source” for the Russian intelligence services, under an arrangement personally approved by Russian leader Vladimir Putin.

Nothing in “How America Lost Its Secrets” is likely to sway Mr....