Book here. From the Yale press site:
John Brown's Spy tells the nearly unknown story of John E. Cook, the person John Brown trusted most with the details of his plans to capture the Harper's Ferry armory in 1859. Cook was a poet, a marksman, a boaster, a dandy, a fighter, and a womanizer—as well as a spy. In a life of only thirty years, he studied law in Connecticut, fought border ruffians in Kansas, served as an abolitionist mole in Virginia, took white hostages during the Harper's Ferry raid, and almost escaped to freedom. For ten days after the infamous raid, he was the most hunted man in America with a staggering $1,000 bounty on his head.
Tracking down the unexplored circumstances of John Cook's life and disastrous end, Steven Lubet is the first to uncover the full extent of Cook's contributions to Brown's scheme. Without Cook's participation, Brown might never have been able to launch the insurrection that sparked the Civil War. Had Cook remained true to the cause, history would have remembered him as a hero. Instead, when Cook was captured and brought to trial, he betrayed John Brown and named fellow abolitionists in a full confession that earned him a place in history's tragic pantheon of disgraced turncoats.