In 1916, while the marooned Sir Ernest Shackleton and two crew members fought fiercely for survival on South Georgia, they had the sensation that another, providential person accompanied them — that they “were four, not three,” as Shackleton wrote. This phenomenon — of sensing a beneficiary presence during extreme tribulation — became known as the Third Man Factor, and everyone from Reinhold Messner and Joe Simpson, to Charles Lindbergh and Aron Ralston, to astronauts and 9/11 survivors has reported it. In The Third Man Factor ($24.95, weinsteinbooks.com), John Geiger expertly blends harrowing accounts with scientific inquiry into the physical, neurological, and psychological elements common to these events. People have suggested myriad causes, from a guardian angel, to lack of oxygen, to an evolved coping mechanism. Whatever the case, Geiger offers a thorough, insightful examination.