“Guantánamo Diary” is the most profound account yet written of what it is like to be that collateral damage. One fall day 13 years ago Mohamedou Ould Slahi, a 30-year-old electrical engineer and telecommunications specialist, received a visit at his house in Noakchott, Mauritania, from two officers summoning him to come answer questions at the country’s intelligence ministry. “Take your car,” one of the men told him, as Slahi stood in front of his house with his mother and his aunt. “We hope you can come back today.” Listening to these words, Slahi’s mother fixed her eyes on her son. “It is the taste of helplessness,” he writes, “when you see your beloved fading away like a dream and you cannot help him. . . . I would watch both my mom and my aunt praying in my rearview mirror until we took the first turn and I saw my beloved ones disappear.”