Based on that history, the government concluded that Mr. Slahi was a “senior recruiter” for Al Qaeda and for a time, listed him as the most dangerous terrorist at Guantánamo. But it has never formally charged him. Mr. Slahi says he left Al Qaeda in 1992, long before it began to target America. His encounter with Mr. Bin al-Shibh lasted one evening and involved no discussion of anti-American plotting, he claims. And Mr. Ressam had left Montreal before Mr. Slahi arrived, and by his account, they never met.

A federal judge who reviewed Mr. Slahi’s habeas petition in 2010, James Robertson, concluded that the government’s evidence was “so attenuated, or so tainted by coercion and mistreatment, or so classified, that it cannot support a criminal prosecution.” The judge said the government’s fear that Mr. Slahi could rejoin Al Qaeda if freed “may indeed be well founded,” but that such concerns did not justify his continued imprisonment. Judge Robertson ordered his release. Despite President Obama’s vow to close the prison, his administration challenged that decision. An appeals court overturned the release order, and Mr. Slahi, now 44, remains in limbo at Guantánamo, where he has been held without trial for more than 12 years.

Mr. Slahi emerges from the pages of his diary, handwritten in 2005, as a curious and generous personality, observant, witty and devout, but by no means fanatical. In the imperfect but vivid English he learned as a fourth language after being sent to Guantánamo, he writes enthusiastically of reading the Bible (several times), “Fermat’s Last Theorem” and “The Catcher in the Rye,” which he says “made me laugh until my stomach hurt.” He came to consider Guantánamo and its staff members his “new home and family,” developing friendships with numerous guards and interrogators, discussing religion, playing chess and watching movies with them. He expresses empathy even for some of his tormentors, saying that “many people in the Army come from poor families, and that’s why the Army sometimes gives them the dirtiest job.”

Though it was written nearly a decade ago, “Guantánamo Diary” arrives at a relevant moment. In his State of the Union address last week, President Obama renewed his pledge to close the Guantánamo prison before leaving office. But the recent attacks in Paris, after the beheadings by militants in Syria, have reignited the anxieties that have kept that prison going for so long.

In such an atmosphere, some Americans may worry: What if Mr. Slahi is simply a clever liar who has successfully hidden his past crimes for 12 years? His book quite effectively undercuts that notion. More important, “Guantánamo Diary” forces us to consider why the United States has set aside the cherished idea that a timely trial is the best way to determine who deserves to be in prison. The overwhelming majority of the remaining 122 detainees have not been charged.

“So has the American democracy passed the test it was subjected to with the 2001 terrorist attacks?” Mr. Slahi asks at the end of his book. “I leave this judgment to the reader,” he adds, noting that “the United States and its people are still facing the dilemma of the Cuban detainees.” Nearly a decade after he wrote those words, the dilemma has not been resolved.