After a group of Qaeda suspects was captured in September 2002, the C.I.A. flew Ali Soufan, an experienced F.B.I. counterterrorism agent, to Afghanistan to help interrogate them. But when he arrived, C.I.A. officials abruptly sent word to keep him from the two most significant new detainees.

Mr. Soufan had made enemies for opposing the C.I.A.’s abusive interrogation of its first prized prisoner, Abu Zubaydah. He eventually won permission to question the two detainees after all, but when he sought to tell the world about those sessions in his 2011 memoir, the C.I.A. censored much of his account as classified.

Nine years later, and after a lawsuit, the C.I.A. has relented. W.W. Norton will republish his book next month under the revised title “The Black Banners (Declassified): How Torture Derailed the War on Terror After 9/11.” Its restored sections add new details to the history of the United States’ early post-Sept. 11 fight against Al Qaeda.

The lesson of the release, Mr. Soufan said in an interview, is “if you fight for the truth hard enough, eventually you will win.”

In the interim, some of what Mr. Soufan sought to discuss has become public, including in the 2014 declassification of a lengthy summary of a landmark Senate study about C.I.A. torture. But his book adds richer details about how F.B.I. interrogators manipulated detainees into divulging information — using rapport-building tactics, he says — and offers a preview of how Mr. Soufan might testify in the delayed military commission case at Guantánamo Bay, Cuba, against five men accused of conspiring in the Sept. 11, 2001, attacks.