I don’t think I’ll be giving anything away by saying what the secret was: Stalin was an agent of the tsarist police, the Okhrana, from 1906 until 1912.
Isaac Don Levine, who wrote the first biography of Stalin published in English in 1931, returned to the subject a quarter of a century later, admitting that his mind had changed. Like Trotsky and other biographers of the Soviet dictator, Levine had heard rumours of Stalin’s treason to the revolutionary cause, but had discounted them. There was no documentary proof.
And then after the Second World War, Levine was handed a document which seemed to offer precisely that: proof of Stalin’s employment by the police as a “mole”. There is a gap of a decade between Levine’s receipt of that document — which he was convinced was genuine — and the publication of this book. Levine’s explanation of that gap is not a credible one, and one imagines that he had some lingering doubts that he might have been handed a forgery. (His later silence on this issue, including in his autobiography, lead me to suspect precisely that.)
Most historians and critics were convinced that this was indeed the case, and the infamous “Eremin Letter” may have just been one more forged Russian document, like the better known “Zinoviev letter” from 1924. But there may be more to it than that.